We know devils make pacts with mortals. We know they offer boons. And that’s fine at a story level. But if you want to represent that mechanically, those details are often buried deep in a supplement or splatbook. The devilbound creature template conveniently appears in the Bestiary 4 right with three other nasty devils you would have looked up anyway. And it’s a good one—easy on the maths (the British spelling seems somehow appropriate here), but with tons of spell-like ability variation based on your choice of 16 signatory devils.
Already a nasty spell-blaster, Carrie Vasterling’s bargain with a pit fiend has now made her nigh-unstoppable. As a dueler, she is simply hardier than the vast majority of her opponents, and this frees her to spend her magical might in summoning monsters rather than bolstering her defenses. Both her aboveboard formal duels and her secret assassinations have managed to create a number of vacancies in the Congress of Mages—vacancies being filled by proxies of Hell who are shifting the voting blocs in subtle but sinister ways.
Guard duty: an easy enough assignment. Even when the prisoner is captain of the house men-at-arms, accused of not only butchering half the royal family in a single night, but of also possibly being the Mustachioed Man, a killer wanted for a slew of murders along the Fogway. It gets tougher when several of the other turnkeys express aloud that the captain should suffer “an accident,” or when the distraught house sorcerer comes down drunk, offering to interrogate the accused with creative applications of ray of frost. But the real drama begins with a simple chambermaid is brought to the cells after curious objects are found in her room. When black tentacles erupt from her body and begin to tear out the bars so that the panicked girl can escape, it is clear that there is at least one more mystery the castle tonight.
Desperate to get another soul for his roster, the imp Skiver made a rather dubious bargain…with an otyugh. Now the poor folk of Wrendale are being tormented by a regenerating, fire-resistant, polymorphing sewer monster that won’t stay in the sewer. Skiver, meanwhile, is tormented by his own impatience—he never expected the otyugh to survive this long. But the beast’s own native cunning (Wis 13) and borrowed invisibility have kept it very much alive, leaving Skiver as eager as the Wrendale folk to see the dung-eater brought down. But the imp needs to watch his back as well. The lemure contracted to answer the otyugh’s summons is not the normal mindless blob of flesh, but a recently demoted barbazu who still retains his memories and intelligence. He is absolutely livid about being an otyugh’s slave (and often its breakfast…because it’s an oytugh) and he will do anything to see Skiver dead.
—Pathfinder Bestiary 4 56–57
Green Ronin’s Advanced Bestiary also has the devil-bound creature template, but that one’s got a hyphen and is, to continue being British, more mathsy.