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This is part 2 of a series of articles about creating custom monsters for 5e. This one covers adapting an existing monster stat block.

Creature Creation, Part 0: Introduction

Creature Creation, Part 1: CR

Refluffing

Not all changes need to be mechanical. I've gotten great use out of simply describing the same stat block in multiple ways. One key enemy of my setting are the Awakeners--beings from Beyond reality that break down reality and try to destroy things. Physically, the common soldiers appear as warped reflections of humanoids, but as if made out of wax that was partially melted and then hardened. Joints in strange places, long spindly arms, etc. They often grow long claws and piercing teeth and use chaotic magic. How did I run them? As slaad (mostly the lower ones). I just described them as above.

This does have its limits--once you find yourself adding or dropping multiple traits to fit the description better, you're probably at the point where you want to formally modify the creature as described below.

Changing existing monsters

One of the easiest ways (beyond simple re-fluffing) to create a new creature is by making small changes to existing published ones.

There are several ways one might do this:

Changing HP

Making a buffed version of a creature can be as simple as using maximum HP instead of average. If doing this increases the HP by more than 30 (90 above CR 19), increase the CR by 1 for every multiple of 30 (this is a change in defensive CR of 2). Note that for creatures with defensive traits that modify effective HP (especially things like resistances that provide HP multipliers) this is a dramatic understatement. If a low-CR creature has multiple resistances, the threshold for a change in CR is about 15 (to account for the doubling of effective HP due to resistances) instead.

The same goes for decreases in HP. You can represent an already wounded monster by having it start below average HP. Unless you dramatically change its health, this usually has little effect on CR and might not be noticeable in play.

Rolling the creature's HP (instead of taking the average) has no substantive effect on CR since the bell curve for NdM is strongly peaked above two or three HD. It is a good (although labor intensive without a dice-roller) method of making each goblin be somewhat different.

Adding HD

Unlike in previous editions, the number of hit dice is completely arbitrary in this edition. It has no effect on anything except by changing the number of hit points the creature has.

By convention, the size of a hit die is set by the creature's size, not its class or type. The following table summarizes the information:

Size Category Hit Die Average HP/die
Tiny d4 2.5
Small d6 3.5
Medium d8 4.5
Large d10 5.5
Huge d12 6.5
Gargantuan d20 10.5


Average hit points are calculated as HD x (Average + Constitution modifier).

When adding PC class levels, add one HD per level, but use the size category's HD size, not the class's HD size. Creatures shouldn't have mixed-size HD.

Adding or changing Traits

There are many traits that can be added without changing the CR; others directly change CR. In all cases the traits should reflect the thematics of the monster. A cringing, cowardly race (like goblins) shouldn't have a trait that grants immunity to fear. Sneaky monsters should have traits that reflect that (and vice versa--creatures with sneaky traits should probably use them). Note that you can come up with new names and descriptions for traits--as long as the mechanical effect does not change there's no change in CR effect. For example, the various Yuan-ti in VGtM have different "fang" traits that all grant a certain amount of damage (of various types) once per turn. This is effectively a modified sneak attack, but the name and the damage type are evocative of the theme of the creature. Another example is the Angelic Weapons trait (add extra weapon dice)--bugbears have the same (mechanical) trait but it's called Brute for them to reflect the differences in theme.

For humanoid (any race) monsters, there is a table in the DMG that includes all the PHB races (plus a few) with modifications. Those generally don't modify CR.

See Appendix A (Traits) for a complete listing of traits from the MM, DMG, and Volo's Guide.

Changing weapons

Monster weapons do not necessarily follow the same detailed rules as players' weapons do. The biggest difference is in what stat modifier is used. While generally melee weapons use Strength and ranged ones use Dexterity, this pattern does not hold for all creatures (especially non-humanoid ones). Natural attacks (claws, bites, etc.) can use whichever modifier is better (like a finesse weapon).

For ease of play, you can use the same modifier for all attacks unless you want to signal a weakness.

Damage dice are also somewhat arbitrary, at least for natural weapons. Manufactured weapons (those found on the PHB table) tend to follow a strict pattern--each size category larger than Medium increase the number of dice by 1; tiny weapons deal 1 damage. As general rules for natural weapons (all for medium creatures):

An increase in DPR of about 10 increases the CR by 1.

Remember to modify the Multiattack action (if present) to account for the new weapons.

Making a solo monster from an existing monster

The bare minimum needed to make a good solo monster is legendary actions. At higher CRs (T3 and T4, mostly), legendary resistances and lair actions become important.

Legendary actions (LA) partially correct for the big killer--action economy. An average party has 4 players, so that's 4 actions to the solo monster's 1. As a result, most legendary creatures have 3 LA uses per round (but only one per turn). These occur [B]after[/B] a player's turn and are recharged at the beginning of the monster's turn. A good pattern is to add the following options:

Spellcasting tends to cost 3 LA uses.

Note that this usually dramatically increases the monster's DPR (since they get 3 more basic attacks per round). The percentage increase depends on how many attacks it makes--this effect is largest at low CR when things are attacking once per round (being a 300% increase in DPR).

Crushbreath (Legendary Ogre)

Crushbreath is a massive ogre who even other ogres are afraid of. His breath is especially bad.

Changes:

Legendary Actions:

New CR: base offensive CR = 8, -0.5 for low ATK relative to CR. base defensive CR = 2, no adjustments. Overall CR: 5. This would be a hard challenge for 4 level 3 PCs or a medium for 5 level 3 PCs.

Note that he would be very dangerous to the squishier party members, especially if he focus-fires one with all his legendary actions. An average wizard at that level has 26 HP, so one full round (main + 2 legendary actions) is enough to outright kill such a player if all hit and deal average damage--action + 1 LA to drop the PC to 0 HP, 2 more (both critical hits) to force 3 death saving throw failures. Crushbreath is pretty stupid, though (at INT 5, so he may not think to do that if the party thinks to insult him (which will drive him into a blind tantrum, smashing the first things he can). He's vulnerable to Wisdom saving throw-forcing effects and will die pretty fast if focus-fired, but he hits like a truck.

Adding PC Class Levels

While this is not wrong, it's highly inefficent and produces wide swings in power. Most of the time, what you really want are the emblematic abilities and traits of a class--just go ahead and add them as traits/actions. The only time I'd really consider this is for creatures that adventure with the party for a while.

If you really want to formally add class levels, here are some considerations.

Adding frontloaded classes make for big power swings--one level of barbarian on a melee-type creature is a huge change (worth a few CR), while adding a single level of wizard to anything but the weakest creatures is basically flavor.

Many monsters only fight once per day (against the party) Generally, PC classes are designed to gradually use up resources over the course of an adventuring day. Giving that same power (especially things like paladin's smite) to creatures that will only fight the PCs means that they can nova without fear. This can pose much more threat than intended.

Spell selection matters Some spells and abilities are really annoying to be used against the players. Hard control (banishment, etc), mind control, force cage, things like this lock a player out of the game for an extended period of time. Talk to your party before deploying these against them. Yes, that's asymmetric, but so is the role of a DM. They don't have other creatures to play while they have to sit there and watch their party have fun. This is especially true if you target them against the player's weak saves or otherwise seem to be specifically targeting someone.

 Evocative traits for classes

In parentheses after the description is the mechanical CR effect.

Creature Creation, Part 3: From Scratch

Creature Creation, Appendix A: Traits