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This is the first appendix for creating custom monsters. It covers commonly found traits.

Creature Creation, Part 0: Introduction

Creature Creation, Part 1: CR

Creature Creation, Part 2: Modifications

Creature Creation, Part 3: From Scratch

Monster Traits

This section reviews the various traits and features found in the DMG, the MM, and VGtM. They are categorized into two major groups--those that affect CR calculations directly and those that do not. Spellcasting, both innate and from spell slots, is reviewed separately as it is complicated.

Each trait is written in the following format:

Name: Effect. Description. sample monsters.

Discussion where it is warranted

Trait names in red text deserve DM caution, as they can be tremendously un-fun if used improperly.

Trait names in blue text are situationally useful. See the discussion.

Costless Traits

These traits do not change the CR of a monster, at least directly. Some of these are strong but niche. Those are noted in blue text.

Amorphous: --. Allows monsters to fit through spaces as small as 1 inch without squeezing. Many oozes.

Amphibious: --. Allows a creature to breathe both air and water. Kuo-toa.

Antimagic Susceptibility: --. Creature is incapacitated by antimagic field and must make a Con save against dispel magic or be knocked unconscious for 1 minute. Animated objects
While this is a major vulnerability against a high level party, at the levels where these monsters usually make an appearance, very few have 3rd or higher level spells to spare. Putting this on a higher-CR monster (such as a golem) would probably result in a reduction of effective hit points--possibly by as much as half.

Blind Senses/Echolocation: --. Creatures with this trait cannot use their blindsight while deafened or in situations where smell is useless. Grimlocks, Bats

Chameleon Skin: --. This grants creatures advantage on Dexterity (Hide) checks. Troglodyte
This trait is situationally powerful. Giving it to a creature that also has ambusher would probably warrant raising the effect of that trait by 100% or so.

Change Shape: --. This grants the ability to change into smaller or humanoid shapes without changing statistics. No combat effect. Metallic dragons

Charm: --. Abilities that fit under this (broad) umbrella impose the charmed condition, sometimes for an extended period. Vampires
Many players find long-lasting mind-control effects to be strongly anti-fun, as they lose control of their character's actions for an extended period of time. Proceed with caution. This ability generally has more role-playing application than combat, and is best deployed against other NPCs (in my opinion).

Damage Absorption: --. This trait specifies a damage type and converts incoming damage of that type into healing instead. Golems
While this is powerful, it's also niche and easily averted. If, however, you put it on a creature facing a group where one or more players is strongly dependent on that damage type (naive blaster sorcerers, usually), it can be obnoxious and feel targeted. I recommend keeping this to energy types (as in not bludgeoning, piercing, or slashing) for both thematic and mechanical reasons.

Devil Sight: --. This trait allows the creature to see through (some) magical darkness. Mainly effective against the darkness spell. Devils (duh), although not the Pit Fiend...

Etherealness: --. This trait allows the creature to enter or exit the Ethereal Plane at will. Night Hag, Succubus
This ability is great for a hit-and-run harasser, but does little in a stand-up fight. If you want a creature that can run away easily, add this. The surprise potential is limited by the fact that most monsters can't handle the party solo. If you put it on a solo-capable monster, it's basically guaranteed surprise and may warrant concern.

False Appearance: --. This trait makes the creature indistinguishable from an inanimate object as long as it doesn't move. Gargoyle, Treant
Like Chameleon Skin, this can guarantee surprise, and so may warrant increasing the effect if you also give the creature ambusher.

Fey Ancestry: --. Exactly the same as the elven trait by this name. Advantage against charm effects, immune to magical sleep.Drow

Flyby: --. Flying creatures with this feature do not incur Opportunity Attacks when leaving an enemy's reach. Giant Owl, Peryton
Here I'm not sure why this is CR --.

Grappler: --. This grants the owner advantage on attacks against creatures it has grappled. Mimic
Again, I'd expect it to actually raise the attack bonus of a creature. Not sure why it's rated so low, especially in conjunction with a constrict-type ability or an auto-grapple trait/attack.

Hold Breath: --. Not quite amphibious, these creatures can survive out of their element for a set duration. Lizardfolk.

Illumination: --. These creatures are their own light sources. Some can adjust the light level at will, others can't. Flameskull.

Illusory Appearance: --. The little brother of Change Shape, this one can be broken by Intelligence (Investigation) checks or by touch. Still no direct combat application. Green Hag.

Immutable Form: --. Immunity to any spell or effect that would change the creature's form. Golems.
Basically stops polymorph in its offensive mode. Very niche.

Incorporeal Movement: --. Creatures with this trait can move through obstacles and creatures for free, but take damage if they end their turn in obstructed spaces. Ghost
This can be strong in a hit-and-run scenario where the creature moves out of total cover, attacks, then moves back into cover. If coupled with Nimble Escape (disengage as a bonus action), it can drastically increase the creature's effective health.

Inscrutable: --. Immune to effects that read minds; attempts to use Wisdom (Insight) against it are at disadvantage. No direct combat application. Sphinx

Keen Senses: --. Grants advantage on Wisdom (Perception) checks using one or more of smell, hearing or sight (which ones, in particular, vary between monsters). Hell Hound, many beasts
Great for guard dogs. Less effective in combat, hence the no-CR effect rating. Foils stealthers pretty well if used right, though.

Labyrinthine Recall: -- The creature can always recall any path it has traveled. Minotaur
I find it hard to create a situation where this trait is more than pure thematics.

Leadership: --. As an action, the creature can add 1d4 to the attack rolls or saving throws of allies within 30 feet for one minute. Basically a limited, non-magical bless effect. Hobgoblin Captain, Knight
This would be better if it didn't take an action to set up. Giving it out as a Legendary Action or a no-action trait would be quite strong, adding 2.5 to all saving throws and attack rolls. I'd rate it as adding ~1 to the CR of each allied creature (0.5 for each of the attacks and saves).

Life Drain: --. This action reduces the maximum hit points of a creature by the amount of damage dealt. Some versions heal the user as well. Wight, Wraith
Creatures with this attack as part of their "best sequence" generally have low DPR and/or cannot use this in their multiattack sequence. Be extremely wary of giving this to a creature with high-damage attacks or adding it to multiattack features, especially at low levels. And absolutely don't add it to a Hydra's bite or a Maralith's sword attack. No, really.

Light Sensitivity/Sunlight Sensitivity: --. These cause the creature to have disadvantage on attack rolls and ability checks in either any bright light or in any bright light caused by sunlight. Light sensitivity is a stronger disadvantage--even a torch provides some bright light. Shadows (Light), Kobolds (Daylight)
On a purely nocturnal creature, or an underground-dweller, these merely change tactics and allow interesting exploits. On a creature that goes into the daylight frequently, they make little sense. Best to avoid them in those cases. Creatures with this trait should probably also have better senses for the dark (at least darkvision).

Magic Weapons: --. These creatures' weapon attacks count as magic. Very few player abilities (stoneskin is the only one I can think of) care about this at all. Balor.

Mimicry: --. This trait allows a creature to mimic sounds it has heard (possibly drawing the players into a trap). Kenku.

Otherworldly Perception: --. This trait allows the creature to sense the presence of invisible or ethereal creatures within 30 feet and can pinpoint them if they're moving. Kuo-toa.
Normally, this is very niche. Against a magic-heavy, stealth-heavy party (especially if they are counting on invisibility and surprise) it can be powerful. Does nothing against non-magical stealth, however.

Reactive: --. This allows the creature to take one reaction per turn, not per round. Marilith.
Coupled with an ability like the Marilith's Parry (+5 to AC vs one melee attack), it's a boost. Similarly, if the creature has many things it can do with its reaction it becomes strong. Best used IMO to make a creature very sticky in conjunction with appropriate OA granting abilities.


Read Thoughts: --. Allows the creature to read the thoughts of a target. Also grants advantage on Wisdom (Insight) and Charisma (Deception, Intimidation, and Persuasion) checks against said target. Doppleganger.

Reckless: --. Just like the Barbarian's Reckless Attack feature. Advantage on attacks that round; opponents have advantage on attacks vs the creature. Minotaur.
Unlike most costless traits, this one has a clear cost and benefit. Advantage is worth ~+4 to ATK; the drawback is ~-4 AC (based on how other similar abilities are statted). In a CR calculation, this will cancel out for a net zero. If, however, the creature has devastating attack abilities (like a heavy life drain), adding this trait gives them a much better chance of landing those attacks (which increases their power). The same goes for creatures with ways of negating advantage/imposing disadvantage.

Redirect Attack: --. This ability allows the creature to swap places with another ally within 5 feet when targeted with an attack. The ally becomes the target instead. Goblin Boss.
This is great for those bosses who like to use living shields (and have no care for the lives of their allies). It becomes useless if there aren't any mooks adjacent, though. One idea would be to give it to a mage with a shield guardian. Combining this and reactive...The downside is that it only works on one attack per reaction and characters can move between attacks. Hence it's most powerful at low levels, before extra attack comes fully online.

Reel: --. The creature can pull any grappled targets up to 25 feet straight to it as an action. Only useful if the creature also has a long-range grapple ability. Roper.

Rejuvination: --. The creature, once killed, comes back to life after a significant period (ie. hours or days). Usually has a "unless X happens" ending clause. Lich.
Great for making a boss that just won't stay dead. No direct combat use, though. Flameskulls are 1 hour, lich-types are 1d10 days.

Slippery: --. The creature has advantage on rolls to escape grapples. Kuo-toa.

Spider Climb: --. The creature can climb any surface without a check and without needing to use hands. This works even upside down. Spiders, Ettercap.
Can be strong if used indoors with high ceilings or walls on a creature with strong ranged attacks. Even stronger if the party is mostly melee-dependent. Otherwise niche. 

Standing Leap: --. The creature can jump as if it had a running start from stationary. Bullywug.

Steadfast: --. The creature cannot be frightened if it can see another ally within 30 feet. Bearded Devil.

Sure-footed: --. The creature has advantage against being knocked prone. Dao.

Teleport: --. Specifically tactical teleport abilities as opposed to the long-range teleport spell. Balor, Blink Dog.

Terrain Camouflage: --. The creature has advantage on Dexterity (Stealth) checks to hide in the chosen terrain. Yeti.

Tunneler: --. Leaves lasting tunnels behind it when it burrows. Umber Hulk.

Turn Immunity/Turn Resistance: --. Reduces the chances that Turn Undead will work. Revenant (immunity), Lich (resistance).

Two Heads: --. Advantage on Wisdom(Perception) and on saves vs blinded, charmed, deafened, frightened, stunned, and being knocked unconscious. Makes sense only rarely. Ettin.

Web Sense: --. Can pinpoint the location of any creature touching the same web as it. Spiders.

Web Walker: --. Unaffected by difficult terrain caused by webbing. Spiders.


Defined-Cost Traits


These traits have CR effects spelled out in the DMG. DPR = Damage per Round. ATK = attack bonus.

Aggressive: +2 DPR. Bonus action dash, but only toward an enemy. Orcs.

Ambusher/Slayer: +1 ATK. Advantage against surprised targets. Useful primarily in conjuntion with high stealth or other abilities that allow surprise more frequently. Although the two have different names, they have the same text. Bugbears.

Angelic Weapons/Brute: +X damage per attack. These add damage dice on weapon hits. X is the average amount added (7 for 2d6, for example). Angels, Bugbears
Does little on primary spell-casters, but increases DPR by X * number of attacks/round for weapon users. Usually included in the attacks for pre-written stat-blocks. Rename this to fit the creature.

Avoidance/Evasion: +1 AC. Evasion is just like the rogue's ability; Avoidance is an upgrade granting the same effect on all saves, not just Dexterity ones. Demilich (avoidance), Master Thief (evasion).
Seems to assume that those saves are infrequent. Against a save-heavy party, this might not be true. This is synergistic with high saves. Be careful.

Blood Frenzy: +4 ATK. Advantage on all attacks vs creatures without full health (which is many/most of them). Sahuagain.

Breath Weapon: +2X/3 DPR. X is the average damage amount. Assume it will be used once, hit two people, and both fail their saves. Dragons.
Non-damaging breath weapons are harder to assess, but as a rule of thumb assign them damage equal to a dragon of similar CR.

Charge/Dive: +X/3 DPR. Deals X additional damage when it hits with a specific attack after movement. Assume it will happen once during the 3 round average. Centaur (charge).

Constrict: +1 AC. The AC boon comes from imposing the restrained condition on one creature. This also applies to any ability that has the same effect--any grapple + restrained trait or ability should get this +1. Constrictor Snake.

Damage Transfer: +100% HP, +HP/3 DPR. This ability means that while attached to another creature, the owner takes half damage and the other half is transferred to the unwilling host. Cloaker.

Death Burst: +2X/3 DPR. When reduced to 0 HP, explode and force save vs X damage. Assume it will hit 2 creatures and they'll fail their saves. Applies to any "explode on death" ability that deals damage. Mephits, Magmin.

Elemental Body: +X DPR. When touched, grappled, or struck in melee, the attacker takes X damage. Assume it will get triggered once per round. Also applies to any other thorns-type ability. Azer, Remorhaz.

Enlarge: +X DPR. As the spell; X usually equals 1d4. Duergar.

Fiendish Blessing/Psychic Defense: +MOD AC. The first is +CHA (always), the second is +WIS (when unarmored and no shield). Cambion, Githzerai Monk.

Frightful Presence/Horrifying Visage/Fear Aura: +25% HP (*). AoE save vs fear. These are all either no-action auras or part of a multiattack routine. The AoE assumes that this reduces the incoming damage by 1/3. (*) Only applies if facing characters under level 10. Legendary Dragons, Ghost, Pit Fiend.
The ghost's version is slightly different (including an ageing effect and requiring a separate action). The banshee also requires a separate action. All the others are high-level effects. All such effects include a "successful saves provide immunity for 24 hours" effect as well. We can extrapolate that a single-target fear effect would give about a 10% increase in health (half as much, rounded down).

Immunity to <type>: +X% HP. Immunity to a damage type. One common damage type or two uncommon damage types are enough to trigger this. Does not stack with more immunities. X depends on tier: 100% in T1 or T2, 50% in T3, 25% in T4.
Deciding where the cut-off is is an art for this one. Be careful of applying them too liberally--instead simply give more hit points and resistance. The burdens of immunities are too unequally distributed across PC classes.

Legendary Resistance: +10*X HP per use. A certain number of guaranteed successes on saves. X = 1 (CR 1-4), 2 (CR 5-10), or 3 (CR 11+). The normal number of uses is 3/LR. Adult and Ancient Dragons.

Magic Resistance: +2 AC. This trait grants the creature advantage on saving throws vs spells or other magical effects. Devils.
This is the most common single trait, especially on fiends and fey. Adding it is an easy way to make beefier monsters, especially against control-heavy parties.

Martial Advantage/Sneak Attack: +X DPR. Similar to the rogue ability. X is the additional damage dealt. Martial advantage does not require finesse weapons and is not triggered by advantage (only by adjacent allies). Hobgoblins.
This is a great way of making a creature feel like a rogue.

Nimble Escape/Cunning Action: +4 AC, +4 ATK (*). Can Disengage or Hide as a bonus action (Cunning Action adds Dash). (*) The bonuses assume the creature can and will hide each turn. Reduce or eliminate them if the creature is not the type to hide. Goblins.
This is the single strongest boost of all the defined-cost ones. It grants about +2 CR (+1 from each of the boosts). It is situationally powerful, however. Without adequate terrain or stat support (can't hide in a white room), it becomes useless.

Pack Tactics: +1 ATK. This grants the creature advantage on attack rolls if an ally is adjacent to the target. Wolves.
I think that this is undervalued, especially for mass mob-type creatures. It's useless on solo creatures, but when used by heavy-hitting minion bosses (an ogre with a mob of goblins, for example) it's pretty much permanent advantage and a significant boost. Another time it would be a big boost is on a heavy-hitting ranged creature who has lots of smaller allies. Fewer monster abilities key off of advantage, though, so it's basically more damage.

Parry: +1 AC. Add X to AC against one melee attack that would otherwise hit. Hobgoblin Warlord, Marilith
Oddly, its value does not depend on X (which seems to be 1 at low CR, 3 at medium CR, and 5 at high CR). Value is limited by only working against one attack, and only in melee. From this, I'd give the shield spell a +3 AC rating. It's more of a boost, works against more things (a full round instead of one attack), but is limited by spell slots. Shield at will is the full +5 bonus.

Possession: +100% health. Take over a new body, become untargetable. Kicked out when the body hits 0 HP, when the creature ends it, or certain spells. Ghost.
This can be brutal. Charisma save (often weaker), removes a player from combat until they're dying. Be very careful with this one. It's probably undervalued here.

Pounce: +X/3 DPR. Pounce is like Charge, except it allows an attack as a bonus action (instead of just extra damage). It usually involves a save vs prone effect as well (with the attack contingent on the target being prone). Tiger.

Rampage: +2 DPR. Allows the creature to move up to 1/2 speed and make an attack as a bonus action after reducing a creature to 0 HP on its turn. Gnolls.
Volo's Guide introduces Aura of Blood Thirst, which allows each nearby creature with Rampage to make a BA attack each turn. This drastically increases the effect of this trait.

Regeneration: +3X HP. X is the number regenerated per round. Trolls.
Several of these have conditions to end the regeneration (trolls and fire, for example). Those that don't have such conditions also only happen when the creature is above 0 HP. If you want it to not die until <condition> happens, use the troll version. Otherwise, use the slaad version.

Relentless/Undead Fortitude: +7X HP. X is the tier associated with the CR: (1: 0-4, 2: 5-10, 3: 11-16, 4:17+). These two traits function slightly differently, but to the same effect. Relentless functions 1x for any attack less than 7X damage that would kill them. Undead Fortitude is a CON save vs 5 + damage, surviving on a success. Wereboars, Boars, Zombies.
The assumption seems to be that it will kick in once for relentless. Zombies will start failing badly if the damage is beyond 7 or so (DC 12). If you tweak the threshold (either amount of damage or the save DC), adjust the bonus accordingly.

Resistance to <type>: +X% health (*). These are your standard resistances--resistance to fire, etc. Only apply it if the creature has several resistances to common types, and it doesn't stack with other resistances. X depends on tier: 100% in T1, 50% in T2, 25% in T3, and 0 in T4.
Where to start counting is a bit fuzzy. I'd apply it if the creature has BPS resistance (against nonmagical) in T1 or T2, but not in T3 or T4 unless it's a low magic world. If magic items are very rare, it probably applies well into T3 if there are lots of martial types in the party. Should it stack with immunities? I think so, as long as it has immunity to common damage types like fire.

Shadow Stealth: +4 AC. These creatures can hide as a bonus action when not in bright light. Shadow Demon.
Although the VGtM trait Shadow Blend (invisibility as a BA in dim light/darkness) is not identical, I figure it should be similar enough for CR purposes).

Stench: +1 AC. Aura--all creatures nearby must save or be poisoned. Immune on a success for 24 hours. Hezrou.

Superior Invisibility: +2 AC. Improved invisibility as an action, duration of concentration. Can cast spells/attack while invisible. Pixie.
I'm not sure how to rate at-will regular invisibility, since it usually burns an action and breaks if they attack. Great for escaping or for ambushing, not as good in combat since combats go so fast (averaging 3 rounds). Gives disadvantage on opposing attacks, and that's about it.

Surprise Attack: +X/3 DPR. Adds X damage against creatures that are surprised. Bugbear.

Swallow: +2X/3 DPR. Creature can only swallow things 2 size categories lower than it or smaller, and usually must grapple the creature first. Acid damage per round, plus blinded and restrained. CON save vs regurgitation if the swallower takes too much damage in a turn. Behir.

Web: +1 AC. One target is restrained (usually at range), STR save to break free. Spiders.

Wounded Fury: +X/3 DPR. +X damage and advantage on attacks when under a HP certain threshold (20% for quaggoths). Quaggoth.
The assumption is that it gets this on one round of the three. Setting the threshold higher would proportionally increase the damage boost.

Difficult Traits to Price


These are all my opinions.

Spell Reflection: ???. These creatures can redirect a spell on a successful save or a missed spell attack. I'd probably increase the AC by 1 or 2, but it strongly varies. Flail Snail.

Petrification abilities: ???. Make sure they take multiple, low DC saves to become petrified (at least at lower levels where greater restoration isn't available). Probably worth some extra AC (as it imposes the restrained condition after the first save or disadvantage to close your eyes).

Instant KO abilities: ???. Better have low DCs, especially if they can happen multiple times per encounter.

AoE confusion effects: ???. If they don't have low DCs or don't provide immunity on a success for higher DCs, they get obnoxious fast IMO.

Indomitable: +? HP. A limited version of a legendary resistance (retry instead of auto-success). Maybe worth half as much?

Spellcasting

And now for the contentious one. What follows is my opinion only. This applies both to innate and normal spellcasting. The difference is if it's spells N times per day or more traditional spell slots.

Spell Selection Matters: Most "pure" spellcasters in the MM are severely under-CR'd as written. For example, the Archmage NPC (even accounting for the spells he has up at encounter start) has an adjusted dCR of only 4.5 and an adjusted oCR of 9, for an average CR of 7. His nominal CR is 12. The mage NPC is also under-CR, with an average CR of 4ish on nominal CR 6. Giving the archmage meteor swarm instead of time stop and his calculated CR jumps to 13 or so. He also has globe of invulnerability, which lets him ignore any caster of level 10 or lower.

The more focused wizard NPCs from VGtM fare better--the conjurer is right on his CR, for example. Spell selection matters. A lot.

In addition, hard control spells (banishment, maze, etc.) are generally considered to be annoying when used against players in excess. They're powerful, but strongly less fun for many players than effects that diminish their output but leave them free to act. Mind control (dominate person especially) is particularly likely to cause a revolt if overused (or uncautiously used).

Opportunity Cost Matters: giving a creature with otherwise strong abilities some weak spells will have little CR effect--they're unlikely to use them in combat. For purely thematic abilities, consider using innate spellcasting as it's much easier to run and takes up less space.

For spells that fix weaknesses but don't deal direct damage (defensive spells on a glass cannon, mobility or offensive control on a slow bruiser, etc), my rule of thumb is to rate it as granting another round of combat (about a 25% increase in DPR for offensive ones or a 25% increase in health for defensive ones).

Buffs, debuffs, and area control spells can be roughly valued as a damaging spell of their same level (there's a table in the DMG). This probably overstates the value, but it's close enough for CR purposes.

Theme Matters: You are not trying to optimize the NPCs to kill the players (except in rare circumstances). NPCs should have spells that fit their fictional purpose and upbringing. Since non-combat spells are CR-neutral, you can be generous with those. Don't always pick the "optimal" spells--that fire beast shouldn't be creating simulacrums or casting wall of ice, nor should the air elementals be raising walls of stone.

Ease of Play Matters: Formal spellcasting increases the options (and thus the load on the DM) considerably. Consider giving custom actions that mimic certain thematic spells instead or using innate spellcasting instead of spell slots. Same effect, much less work to run. Recurring NPCs might need more fleshing out, as do those that have narrative value as individuals outside of combat.

Remember that you don't need to give spellcasting levels to be able to cast spells. You can just say "this NPC can cast fireball x/day using CHA" or "Action: Fireball (recharges 5-6), DC 12."

Summoning


Summoning does not seem to have a fixed CR effect, nor a particular pattern throughout the MM and VGtM. Two (very rough) suggestions for balancing it:

Balance based on DPR: If the creature disappears once the summoner dies, try adding the summoned creature's DPR to 2/3s of the summoner's DPR. The 2/3s accounts for any actions needed to summon them and the possibility of not lasting the full duration.

Balance based on XP: Alternatively, round the summoner's calculated CR down (if not exactly on an existing CR) and add the XP for the summoned creature to the XP for the summoner. Use this XP total to generate the CR for the summoner/summoned pair.

Devils (whose CR tends to be a bit high anyway) don't obey either of these suggestions when using the variant devil summoning option.