Demon Contracts for OSR games

 Art and gifs in this post is from Atlus' Shin Megami Tensei series.

Howdy y'all, here's just a little random thing I came up with while working on a bigger announcement (yes yes you'll see more soon!)

So you know by now that I'm a bit of a Shin Megami Tensei/Persona fan, and recently I've been playing some OSR dungeon crawling with a group of friends.
Part of me has always felt like there were several points in common between SMT and OSR - starting with dungeon crawling, and up to weird fantasy designs and an overall brutal and bleak perspective on reality. Not to mention the whole Law vs Chaos thing, of course.

So today I present you... my rules for having demon contracts in OSR games!

No guarantee your demons will be this cute, tho.

Let's go!

The Core Assumption: Demons (or Shadows, or Summons, or whatever) are largely based on Hirelings/Followers, and on the way these characters function within your system of choice.

Contracts
Demons are more powerful than normal hirelings, and thus each Demon must be bound by a Contract. You can only command a Demon if you hold their Contract, and each Contract takes up one space in your backpack (if you use weight instead of inventory space, it's about the same heft as a book).

Maximum Number of Bound Demons
Much like hirelings, Demons are limited. At any time, the maximum number of Demons you can have bound is equal to your Wisdom modifier (minimum of one). If your system doesn't feature Wisdom, any similar attribute that can range from 1 to 4 or 5 and is tied to awareness, sixth sense and communion with unseen forces will do.
If your character is an occultist, you might want to tie their maximum number of bound Demons to Intelligence: this can also make sense. It all depends on whether you want demon summoning to be tied to "lore and study of formulae" (INT) or to "being smart enough to trick demons and keep them in check" (WIS).

Demon Stats
Demons follow whatever rules creatures follow in your game, but a few special rules always apply: 

• A character can bind any Demon with a number of Hit Dice equal to their Hit Dice or lower: For instance, only a level 3 character will be able to bind a Demon with 3 HD.
• Demons will not carry items: Even if the Demon normally appears clad in armor or bearing weapons, these are "part of their body".
• Each Demon has needs: And they will communicate these needs when the contract is signed. If a day goes by without these needs being satisfied, the binder must make a Saving Throw versus Magic (or Spells). On a failure, the contract crumbles to ash and the Demon is free. They might also be pissed.
• Demons do not level up via XP: Instead, they might draw benefit from sacrifices and offerings. These are determined by the GM on a case-by-case basis.
• Each Demon knows a number of spells equal to their level: These can be random spells, or spells that fit with the tradition tied to that demon - such as Ukobach being able to cast fireball since he's a demon associated with flame. The Demon can cast these spells at-will, but this doesn't mean they'll do so for you.
• Each Demon will generally obey the orders of the binder, as long as the binder carries the Contract. However, the Demon will interpret commands in a way that follows the Demon's alignment and personality. For instance, a cruel and chaotic demon will always attempt to sow disorder with their actions.
• A Binder can demand up to X spells from their Demons every day, where X is equal to the binder's level. This amount resets at midnight. Note that this number remains unchanged regardless of the amount of Demons that character is controlling: if you are level 2 and control four demons, you can still only demand two spells over the course of the day.
• Whenever the binder asks a Demon to cast a spell beyond the daily limit, that Demon (through the GM) will consider whether that use of the spell meets their favor: then, the binder must make a Saving Throw versus Magic (or Spells). If the use of the spell is something the Demon would enjoy, the binder has advantage on the Saving Throw.
  On a failure, the Demon is infuriated and cannot be summoned until past midnight.
  On a success, the spell is cast but the binder immediately loses an amount of Hit Points equal to 1d6 + the level of the spell.
  If this reduces the binder to zero Hit Points, they are claimed by the Demon and carried away into the "other world", alongside any gear they had on themselves. All Demons claimed by the character are immediately freed from their contracts and may wreak havoc as they see fit.
• If a Demon is reduced to zero Hit Points, they vanish immediately. They can be summoned again after midnight. Particularly powerful Demons might specify in their contract that they reserve the right to rescind the contract if they are treated poorly - which might include falling to zero HP.
  Again, this is something the Demon will mention when the contract is made.

How to write a Contract
This isn't something I'll give you rules for. Every Demon will have different needs, specify different conditions, and so on. Research the demonic tradition and identify the key aspects of the entity: Agares might prize contact and familiarity with the earth, Forneus might appreciate rhetoric and social occasions, and so on.

Maybe one day I'll compile a list of demons in a format such as...

Decarabia (3 HD, 15 HP)
Appears as a pentagram star, changing into a man under the conjurer's request.
Attacks: Star ray +3, d6 damage
Spells: Detect Magic, Identify Herbs and Poisons, Change Shape (self only, can change into a bird or into a human)
Contract Conditions: Decarabia demands to be exposed to new knowledge, if summoned.
Alignment and Personality: Chaotic, will always prioritize what piques his curiosity over what the Binder needs.

But before that, you can still find a hundred weird summonable entities here in this Coins and Scrolls blog post!