How to decide the level of new monsters? [closed]

Question

I'm running a D&D sandbox (with my own variant of D&D, of course, but it is reasonably compatible with old versions of D&D and various retroclones), and I use several adventure modules and random tables that are made for various rule systems (not all of them level-based), and sometimes for no system at all. In this situation I often have to create game statistics for various creatures and non-player characters.

We play so that the players are responsible for their success. The referee uses pre-created material, random tables, established rules, and (when necessary) improvisation to run the game. The referee is (as much as possible) a neutral referee, and does not for example choose how difficult encounters are, what is encountered, or how hostile they are.

For this reason it is important to be able to establish game stats for a creature without thinking about how challenging the encounter is or should be. This is easy for everything but monster hit dice or levels, which are fairly disassociated from the fiction.

How can I assign hit dice / level to a creature based on its fictional description only?

Something like Dungeon World monster creation or Burning wheel monster burning is okay, since they rely on the nature of the creature within the fictional world. Pathfinder monster creation is completely unsuitable, since it starts very strongly by deciding the CR of the creature, and CR is not a fictional detail of the creature.

I am looking for both published methods and methods people use in their house games.

A good method would be one that can be used on-the-fly during play, with all the time constraints that implies. This is not necessary, but would be nice.

Some examples

Two out of three adventurers died when the third one abandoned them (and wizard locked them in with a murderous undead thing). They'll rise to plague the traitor as corporeal spirits of vengeance if someone is foolish enough to open the wizard locked door. What level should the vengeful dead be? (Both characters were first level when they died.)

Failed spellcasting resulted in a character summoning an angel, bright as anything, with a burning sword and four wings. I also have game mechanical stats for the seraphim, but they are for Burning Wheel (in the Monster burner). What level should it be?

Answer 1

I'm assuming Hit Dice that work like in D&D 3.5 here, because I'm not familiar enough with older versions. I hope they're similar enough that this holds up.

Racial Hit Dice

These represent how tough a creature naturally is. Any creature of the same species will have these simply by being (an adult, healthy) member of that species. They are based primarily on the size of the creature and the build of the creature. In order to decide how many of these a creature gets, start by looking at the creature's volume. (not its in-game size)

The reason to look at volume and not in-game size is because creatures of the same in-game size could have completely different volume. Compare, for example, a giraffe and a rhino. Both creatures would take up about the same area on a playing field, but the rhino is much bulkier and should have a higher base-line HD for size.

Afterwards, check to see if there´s anything about the creature that would make it be even tougher than its bulk suggests. For example; being made of pure rock for example, or being undead and thus having no organs that can fail would give a creature even more HD for its bulk.

Likewise you can come up with reasons why a creature might have less HD for its bulk; being exceptionally fragile, such as a light bird, ghostly creature, being made of air, etc.

You can probably come up with a table of guidelines per size for what would be acceptable base ranges, although some creatures might go off the charts.

Level

After assigning the base Racial HD that any healthy member of a species would have, sometimes you want to add additional dice that are unique to that creature and represent how the creature is more exceptional than others of its kind. These often come with other cool perks based on what the creature does or has learned, but they don't have to.

The thing with Level is that you can't really see level; they are part of the fiction and that they say something special about the creature's knowledge, experience, and training, rather than saying something abouts its look, size and composition.

The only real guidance you have here is the creature's personal story and the only way to objectively assign the right number of levels is to think about the impact that creature's actions have on the story. For example, a creature that the player's have been hearing about for the past 5 sessions probably has a lot of Level dice, while a creature that just randomly came up today probably has only a few. If lots of people in the area know about and warn you for a creature, it probably has more dice than if nobody is really aware of it or considers it a problem.

Coming together

So basically to assign a creature the proper number of hit dice/level, first think about what the creature looks like, is made of, its size, natural toughness, etc to determine the base power-rating for such a creature. Then add to it what the creature has accomplished, learned and the reputation has built up, and use that to determine how many extra levels this specific creature should have.

Closing note

If you're going to use this system to come up with random, objective information it helps to be fair to the players as much as possible. Springing a super-high level bandit on them because the dice decided is a little weird because they would have to have heard of this guy being in the area unless he literally just arrived yesterday (in which case, the encounter should reflect this in poor choice of ambush location, maybe over the top introductions, limited or unfamiliar henchmen, etc)

If you're going the random route, remember that the world is alive and that news spreads. This is something that random tables usually do not take into account, but really ought to, because it helps make more sense of the world.

If one of the options on the random encounter table is "Red Dragon" then there's a 99% chance that someone you pass on the road will warn you about said dragon being in the area.