This is part 0 of a guide to monster creation. This one covers the philosophy--what makes a good monster? What is the role of a monster in D&D?

For Part 1, see Creature Creation, Part 1: CR.

The role of monsters in D&D

By monster, I mean any NPC that is likely to meet the PCs in combat. You rarely need to stat up the village blacksmith or the inkeeper as full creatures but you do need to know how that goblin or that enemy mage will act in combat.

Monsters are foils to the PCs; they have different but complementary roles. For one thing, there generally are a lot more monsters than PCs in any given area. For another, only a small fraction of monsters live beyond a single combat. Recurring NPCs are relatively rare by comparison. On the other hand, PCs are expected to undergo attrition over the course of multiple fights between long rests. This means that monsters, in general, must have lower nova power than a PC--there's no resource management minigame for monsters. Because PCs have significant nova potential,monsters also tend to have higher health to allow for combats longer than a single round.

In addition, the DM has to be able to handle a bunch of monsters simultaneously, while each player only handles their PC. This, coupled with the throw-away nature of monsters generally, makes monsters out to be much less complex than PCs. PCs can have lots of moving parts, resource pools, recovery mechanics, non-combat ribbons, etc. Monsters much less so.

In fact, the non-combat part of most monsters is almost completely up to the DM in 5e. Do you want that evil vizer to be great at persuading the king? He is, you don't need to give him explicit permission slips (features) that let him do that. The blacksmith can simply make things; no need for 52 ranks in Craft Weapons (and the concommitant levels and combat potential).

The last difference is one of optimization. Don't fall into the trap of over-optimizing your monsters against the players. D&D should not be antagonistic, with the DM trying to "win" vs the players or vice versa. You control the world--try to make it as organic-seeming as possible. This means having monsters with flaws, people who do sub-optimal things (or who learn bad but thematic spells, for example). Colorful and easy to run are much more important attributes than are perfect tactics or optimized builds. 5e is loose enough, with few enough game-breaking tactics and builds, that you rarely need to do such things. Good tactics certainly help in an encounter, as does good terrain, but the most important thing is having fun. Know your players--if they want difficult tactical combats, give it to them. If they want colorful descriptions but don't care about always being on the edge of death, give that to them. Etc. You win (as does everyone) when people have fun. Each group's fun will be different.

What makes a good monster?

As I see it, the characteristics of a good monster are these (in no particular order):

• Easy to use. Complicated monsters with lots of moving parts will either not use those parts (meaning you wasted your time adding and balancing them) or will take lots of table time (slowing things down and boring players). Except for legendary solo monsters, remember that you're going to be running multiples of most of these monsters (or groups of 3-8 total creatures normally). Contrary to common belief, adding spells to a monster doesn't make them more interesting. It just takes more time and brain space to run, leading to worse tactics.
• Thematic. Creatures should have strong thematics that link them to a place in the world. A ice-and-snow creature in the middle of a barren desert is unlikely; a massive flying bird underground is also a bit odd. Same goes for abilities--your descriptions should hint at the strengths and weaknesses of the creature. Creatures covered in massive armor plates or scales should show those; things without mouths shouldn't have a bite attack. And the list goes on. Tactics, capabilities, and descriptions should all work together.
• Properly balanced. Glass cannon PCs can work. Glass cannon monsters rarely do--this edition doesn't handle rocket tag (first one to land a blow wins) combat very well. Same goes for the reverse--a wall of meat that doesn't do much is pretty boring (this is often called padded sumo combat). Monsters tend to have more health than a comparable PC but lower burst damage output. Being dropped to 0 HP right out of the gate with no chance of response is a bad outcome for many players--you end up spending combat doing nothing. Same goes for hard CC--the player gets to spend their turns doing nothing and watching instead of playing. Favor abilities that hinder PCs instead of just no-selling/stopping them. Same goes for damage resistance vs immunity--immunity should be pretty rare and very thematic. Resistance is fine, however (basically acting as a health buffer).

What's to come?

Creature Creation, Part 1: CR covers the actual mechanical calculation of Challenge Rating (CR).

Creature Creation, Part 2: Modifications covers ways you can alter existing stat blocks instead of starting from scratch.

Creature Creation, Part 3: From Scratch covers the steps needed to make good monsters from the ground up, including using PC classes.

Creature Creation, Appendix A: Traits covers the published traits and their effect on CR and on balance generally.

Creature Creation, Appendix B: Templates gives a small list of templates--bundles of traits and abilities you can give monsters to enforce a theme. Some are from the published books, others are homebrew.