Demon Pact Magic
Demons do not have supreme power over the world. Truthfully their powers are quite limited. What they do excel in is striking a deal. Demons take on many forms and how they go about sealing a deal should reflect that. A demon of war through conquest, of lust through seduction, and so on. They are renowned for how hard they will strive to accomplish a deal no matter how many parties to an agreement they must get in order. Demons do not deal in conditionals, requests must be absolute. The price is always their freedom.
They may only beseech, never command.
Example Demons
Task Imp - 4 HP
Demonic Taskmaster - 9 HP
Demonic Titan - 24 HP
For every deal that must be made to make the initial request happen add a d6. Roll all the dice as damage against the demon's HPs. If it survives, the task is finished without anything else asked of the players. If it "dies", the players inherit the problems of the other deals the Demon made to get this done.
A disgruntled farmer asks for the power to cast fire from his fingers. A lowly imp goes to the Crucible of Flame to ask for primordial fire. The foreman agrees but needs a lighthouse to be burnt down since its lantern is made of an unsightly artificial light. A d6 is rolled dealing 2 damage to the imp. The imp completes the deal and the farmer now casts fire from his fingers.
A hedge wizard tasks a demon to deliver a letter to his lost mentor. The demon asks the Lord of Owls if he has seen the mentor recently and he has! But the Lord of Owls needs a den of foxes removed from its forest. A d6 is rolled and the demon takes 3 damage surviving this barter. The demon drives away the foxes and the wizard's letter is delivered.
A wounded knight asks a powerful demon to mend his dying body. The demon goes to Death to ask to stay his cold scythe. Death demands another soul of equal power. The Demon asks Goblin Spearmaster to ambush and kill a knight of renown. The Spearmaster agrees so long as the Demon can pluck the eyes out of a profane goblin cleric. The Demon asks a scheming initiate to carve out the priest's eyes. The initiate agrees. 3d6 are rolled doing 16 damage, the demon is unable to finish the tasks. The wounded knight is brought back to full health but now must fulfill the other deals that were struck to give him this new lease on life.