Arkham Horror 3rd Edition: Some events offer an investigator a Dark Pact, often in exchange for some major benefit (sometimes survival). The Pact has a 1-in-6 chance of coming due at each Reckoning, and only then is the cost revealed — which can be anything from instant death to a flood of monsters.
Diabolists are those who have sold their souls to Hell for power. They gain an infernally-themed version of the Purposely Overpowered Mythic Companion template, unlocking general character improvements and Black Magic abilities.
The Order of Hermes strictly forbids any dealing with demons whatsoever, on pain of death. This applied even when one mage and a demon agreed to leave each other alone: the Order takes no chances with creatures that are immortal, intrinsically depraved, and compulsively treacherous.
The Devil's Bargain mechanic in Blades in the Dark and its derivatives allows this trope to happen both literally and metaphorically: the players can get a free die on any action roll if they accept an additional negative consequence that will result from their action even if the roll itself succeeds. This complication can range from a literal deal with a devil ("devil" being the setting's term for any powerful supernatural nasty) to something mundane, like owing a favor to a rival mobster.
The Acamoth make deals with mages (and possibly other mortals) whereby they consent to allow the acamoth to enter their souls and Mind Rape them in exchange for granting them powers. As the acamoth are generally concerned with inscrutable, long-term goals (i.e.: conquering reality), they rarely care to take a soul outright, preferring payments with more useful and lasting dividends.
The Lethean is a specific Acamoth who offers to take away painful memories or things you'd rather not perceive. The cost is that you're giving up parts of your memories and perceptions.
The Oath of Ruin is an Abyssal entity who offered one guy Awakening in exchange for making his bloodline its foothold in the physical world. It will manifest once he has enough living descendants, and is outright stated to only be banishable by doing something truly evil).
The Scelesti are mages who twist their souls towards the Black Magic of the Abyss, generally through pacts with Abyssal entities. The most powerful bargain with the Abyss's emissary for the Elder Diadem, a soul mark that grants potent Wrong Context Magic but grants the Abyss absolute control over the mage's soul.
Changeling: The Lost: A Changeling or True Fae can bind a human to a Pledge, offering money, power, or other benefits in exchange for various favors - but most Changelings, due to the importance of contracts and pledges among the fae, are geniuses at twisting the meanings behind the words of their pledges. Not only that, but an agreement with the True Fae has a better-than-average chance of ending in the human kidnapped and subjected to Mind Rape in the True Fae's Arcadian realm. Best case scenario: he escapes as a Changeling. Worst case? You don't want to know.
Hunter: The Vigil and World of Darkness: Inferno also include rules for making dark contracts with demons; some of these are the traditional "sell your soul" variety, while others simply require you to perform actions that feed the demon's Vice. Many of these demons are actually remarkably straightforward in their dealings, and hunters often find themselves searching for loopholes and hidden clauses that aren't really there.
Demon: The Descent has these, naturally. Demons effectively make soul pacts with mortals, offering things in returns for aspects of the mortal's existence (a job, a significant other, etc.). Usually those are pretty benign, only resulting in others remembering the demon participating in whatever it was instead of the mortal, allowing the demon to adopt the Cover, allowing them to pose as a mortal to thwart agents of the God-Machine. A number of demons go for a kind of pact that's only cashed in if the demon's Cover is destroyed entirely... but if that happens, the demon can just adopt his thrall's existence instead. And the thrall vanishes. Utterly.
In The Dark Eye there are twelve Demon Lords as well as several powerful so called free demons one could forge a pact with to gain gain access to special powers depending on the lord and being able to summon his or her servants more easily. The downside, aside from eventually going to the hells, is that over time anyone with a pact will start developing character traits that fit those of his demonic liege. So a person with a pact with the Lord of Strife and Movement will develop a Chronic Backstabbing Disorder and soon won't be able to stand still or even sleep anymore, while someone with a pact with the Lord of Slaughter and Bloodshed will become more vicious in battle and start developing wounds that always bleed but do not weaken him.
Quite common in Deadlands. Nearly all magic comes from the spirits of The Hunting Grounds. The nice ones are in the minority. The most common playable version of this is The Huckster, who has to allow a demon into his body in order to cast a spell. Just hope the Huckster isn't tricked into giving the demon too much control.
Used in Dungeons & Dragons, where demons and devils can be summoned to make pacts with mortals, and can grab your soul and run if you mess up the pentagram or leave out the "Promise not to kill me when you're done" clause in the contract.
Fiendish Codex II: Tyrants of the Nine Hells has a whole section detailing Faustian pacts. Devils prefer to strike a Pact Certain with a foolish mortal willing to explicitly sell their soul to an archfiend, but more commonly use the Pact Insidious. This approach doesn't directly damn the mortal to Hell, but they must trade a service for the devil's offer of assistance, treasure or magical powers. Many mortals believe that they can reap the rewards of such devilish pacts without suffering the consequences, unaware that each corrupt act they perform in exchange is another step towards damnation. The devils, meanwhile, usually make arrangements for their "partner" to die as soon as their soul is marked for Hell, to avoid any chances of atonement or deathbed repentance. There are only two grounds for exemption in these pacts: if the mortal was coerced into them, or if the devil didn't deliver the goods as promised. A sufficiently knowledgeable character can in fact protest along these lines and take their case to an infernal court, and since devils are as Lawful as they are Evilinvoked, they will offer the plaintiff a fair trial, even providing legal counsel as needed. A combination of Knowledge (the planes), Diplomacy and Perform (acting) checks can see a mortal's soul freed from a devilish contract... but even if the mortal wins their case on merit, it's possible that their other actions have damned them to the Nine Hells anyway. "Much diabolical laughter then ensues."
5e's Mordenkainen's Tome of Foes details the kind of bargains each Archduke prefers to strike (or, in some cases, is limited to). Zariel offers martial prowess, Dispater trades in information, Mammon acts as a Morally Bankrupt Banker, Fierna offers people skills and a talent for manipulating hearts and minds (her co-lord Belial doesn't strike deals, re-directing them to her), Levistus grants a one-time save from danger, Glasya offers her Rules Lawyer talents (she's often called on to get out of deals with other devils), Baalzebul offers redemption from personal humiliation, and Mephistopheles offers magical knowledge.
Even deities have fallen victim to devilish bargains. The baatezu's creation myth (which you can believe at your own peril) states that they were once angels who battled demons after the dawn of creation, but were corrupted by the evil of their enemies until they were as twisted as their foes. The gods wanted to kick them out, but Asmodeus, the leader of these fallen angels, successfully argued that they had only been doing their duty, and there was no lawful justification to exile them. When the deities moved on to create mortal races, but were horrified when some of those foolish mortals trafficked with demons to cause destruction on the Material Plane, Asmodeus offered his fallen angels' services to punish mortals for their transgressions. The gods agreed, but found that having a bunch of fiends around tormenting mortal souls ruined the atmosphere of the heavens, so Asmodeus made another offer: he and his devils would relocate their soul-punishing operation to the Nine Hells, on the condition that the devils would be allowed to sustain themselves on magic drained from the tortured souls. Thus was struck the Pact Primeval, granting divine approval to the operation of Hell. While the gods were pleased to be rid of the devils and their ghastly tortures of mortal souls, they soon realized that fewer mortals were arriving in the heavens, and were horrified to find that the devils were actively tempting mortals into evil in order to increase the devils' power and numbers. When the outraged gods confronted Asmodeus about this, he merely smiled and said, "Read the Fine Print."
Demons are generally too Chaotic Evilinvoked to make such complicated, long-term plans, but the demon lord Pazuzu is known to take an active interest in mortal affairs. Anyone who says his name three times grabs his attention, and after telepathically scanning them, the Prince of the Lower Aerial Kingdoms will teleport to them and offer his aid. Any who accept it will shift their alignment one step closer to his, yet the notion of a demon lord for an ally is rather tempting, especially since Pazuzu is canny enough to take care that no evil emerges from his assistance (at least the first time).
The Glabrezu demon is also rational enough to make deals, and likes to amass personal power in hopes of tempting targets to summon and make deals with them. Such power tends to run out at convenient moments to get the summoner killed and damned to the Abyss.
Zig-zagged by the Warlock, a spellcasting class that gets its powers from pacts with extradimensional entities. That said, they don't have to be evil (though they indeed can be), and the creatures they deal with don't have to be devils (though they indeed can be)- warlock pacts can be made with demons, fey, creatures from the Far Realm, subterranean horrors, sorcerer-kings, and so forth. Such powers are generally evil or at least alien, but again the warlock is not automatically evil for making these pacts, and can actively work against their patrons with their own powers. The 3.5 version also mentioned that the deal doesn't necessarily have to be made by the warlock themselves — some deals could be inherited and remain dormant for a few generations until some poor fellow gets saddled with powers from a deal he never knew about. The 5E version, in addition to having outright and archetypal Good patrons, also emphasises that it is possible to con the patron and end up with a pact that obligates them to provide power in exchange for the warlock keeping their soul and being free to do what they want. Furthermore, it doesn't even need to be your soul that you used as payment; Mammon, Archdevil of greed, accepts ordinary freaking money for example (granted, enough money to bleed several dragons dry, but still).
Binders are accused of this, and practice what is called Pact Magic, but it's not a traditional deal with the devil. The Vestiges they call up are fragments of sentience beyond normal conceptions of life or death, and the bargaining process only determines whether they will be able to influence the Binder's behavior in quirky ways before granting their Powers via Possession.
The sourcebook Evil, true to its style, contains some rules for how to handle deals with devils and demons, including a list of benefits (and their downsides) that you can get, typically in the lines of "get superpowers but look and/or smell demonic."
Van Richten's Guide to Fiends for the Ravenloft setting contains part of the actual text of such a deal, between a human man and a succubus (for sexual favours, of course). It sounds a bit like a typical software license agreement a computer user would agree to without reading, though in this case the man accidentally doomed his wife to be murdered by accepting the part involving the removal to any obstacles to the deal, including the elimination of "any organic organismsnative to the environment in question."
Hags also love making such deals, using the dark magic they collect over the course of their long lives to offer bargains to mortals. Unlike devils, hags don't ask for the dealer's soul- they just do it to see people suffer, so they'll just be the biggest Jackass Genie possible.
The Gates of Hell describe the details of such deals for most of the Hell nobility featured. The book also includes the catches. For example, one devil can give bonuses to fighting skills. However, as you get them, you get penalties in other areas. At the end, you must fight an Evil Twin of yourself, the twin having the bonuses, but you only having the penalties. If you somehow manage to win, you get more bonuses, but then have to repeat the same fight, except the Evil Twin has more bonuses, and you have more penalties. Eventually, you have to fight the devil himself. No one knows what happens if you defeat him, since that never happened.
Overall, such deals can be used by Dungeon Masters to bring Player CharactersBack from the Dead, and to introduce new plot threads while they're at it.
Eberron: Some Cults of the Dragon Below spring up through some kind of bargain with emissaries of the setting's various canned evils, whether that be mages in Arcanix dabbling in the dark magic of Sul Khatesh or dwarves in the Mror Holds taking up daelkyr symbionts and secrets for the War Below. Typically, some level of unreality begins to seep in: Sul Khatesh's most notable cult, the Court of Shadows, is strongly associated with cultists becoming increasingly obsessed with hallucinatory intrigues against other cultists.
Exalted has a number of versions, seeing as tacitly summoning and bartering with demons isn't exactly frowned on by the religion of the Realm, but the most traditional might be Abyssal Exaltation. One of the Deathlords comes to you on your deathbed and promises you a second chance at life, as well as power beyond your imagining... as long as you'll bind yourself to his service, throw your name into the Void, and aid him in the destruction of Creation.
And then there are the more traditional devils, the Yozis. In Second Edition, one of the ways the Yozis recruit the Green Sun Princes is to go to a mortal who had a chance of performing great deeds that would have lined them up for Exaltation, but backed down in the face of adversity. The Yozis then offer them a second chance at greatness. In Third Edition, this has been modified so the Yozis choose candidates who are oppressed, downtrodden, and extremely likely to be pissed off at Creation, giving them a chance to go apeshit and upturn the status quo.
And then there are the akuma, Exalts and "enlightened" mortals who threw their lot in with the Yozis. The resultant procedure gives them great power, rapes their free will out of existence, and turns them into a very intelligent oven mitt with a daiklave. Yeah, becoming akuma is rarely a good idea.
Among the Green Sun Princes, the favored of Cecelyne can make these and dictate terms thanks to the Verdant Emptiness Endowment Charm, which allows them to answer wishes made even in jest. Further Charms allow them to grant further benedictions and heal the grievously wounded... as well as revoke their blessings, painfully, if payment isn't offered up.
The fan-made "Terrifying Argent Witch" remake of the Lunars enables them to play the part of the Devil; fulfilling a spoken desire (serious or not) in order to "buy" someone's shape (and possibly soul, given how Lunar shapeshifting works), sealing oaths in blood (complete with a terrible curse should they break the pact), lending power at the cost of being emotionally tied to the Witch in question.
Godbound: The Shackled Court of Uncreated Night have a particularly clever spin. Unlike most versions, which just shaft everyone who takes the offer, they periodically allow a mortal petitioner to get everything she wants without nasty little tricks or hidden costs. That way, when the petitioner runs off into the world boasting about how they managed to trick the Shackled, more people are willing to give it a shot themselves...
Hoodoo Blues is a horror rpg based on Southern folklore, where one of the main character types is a "Crossroader". Someone who has made a literal Deal Withthe Devil for supernatural abilities. How much power they are given depends on the exact deal made. Most crossroaders have a clause in the agreement that lets them keep their soul and powers; usually by completing a near impossible task, or performing a service for the Devil. Those with the most power have outright given up their soul with no escape clause. The only way to keep their soul when/if they eventually die is to find something more valuable in exchange (the Devil always grants Agelessness as a freebie, so they at least have time to figure a way out of the deal).
The Lilim's primary schtick is to offer to fulfill a Need for another being — a human, another demon, an ethereal, a particularly luckless angel — in exchange for a binding Geas. Humans usually deal with their undercover Roles, unwittingly giving away the keys to the backdoors of their minds in exchange for that ever so helpful advice, service, or off-the-books problem solving. Celestials and ethereals know better, so the Lilim usually come to them only when they're so desperate that they have no real choice in the matter. Of course, in all cases, Lilim are perfectly happy to subtly arrange for someone to find themselves dealing with some kind of insurmountable problem just in time for a helpful Tempter to swoop in and offer to fix things... for a price.
Due to his focus on business and crooked deals, Mammon has a particular focus towards drafting supernatural contracts and suckering others into signing them.
When he was still a Word-bound demon, he advised his Prince, Asmodeus, to bargain with Lucifer for the right to claim any soul that signed itself away to him in addition to the basic quota all Princes claim. When he became a Prince himself, Mammon again talked the Devil into granting him this particular largesse. Humans who bargain their souls to a Miser don't automatically go to Hell — it's ultimately just paper, and they need to be damned the usual way — but, on passing the gates of the Pit, anyone who entered such a contract is automatically claimed by Mammon, and he emphasizes to his servants that they should offer such deals to as many mortals as they can and then follow through to make sure that their contractors then head directly to the Pit. The highest invocation modifier for attempts to summon him to Earth is in fact a binding contract for a human's immortal soul, freshly signed in blood.
This extends to the powers granted to his servants. His Balseraphs can place an illusion over legal documents to mask what they say, only revealing their true contents when the desired name is safely on the dotted line. His Knight of Treasure distinction allows demons of this rank to trick reality into thinking that a target has signed something they didn't — for a time, at least. His Art of the Deal attunement allows his servants to, after talking aloud with someone about a deal and agreeing on terms, spontaneously generate a matching contract by reaching into a pocket or briefcase and producing the agreed-on document with a flourish, ironclad and ready to sign.
"If a duplicate or triplicate of the contract is required, it can be produced with equal aplomb."
Magic: The Gathering: This is a large part of Black Magic. The most up-front about it are the demons you can summon and involve a "payment", a drawback in the form of having to sacrifice other creatures, discarding cards or losing life. Note the flavour text on the latest reprint of the classic Lord of the Pit: "My summoning begins your debt, planeswalker". The character Liliana Vess has a major deal with dark forces in her backstory.
A lot of cards involve making some payment (usually life) to some dark force to gain some benefit (usually cards), for example Sign in Blood, Vampiric Tutor, Death Pit Offering, Tainted Pact, Yawgmoth's Bargain, etc., but the game's most egregious example however, remains Contract From Below. A long out-of-print card that used the infamous "ante" mechanic of the game's early days. It allowed you to draw a fresh hand of 7 new cards, but required you to bet a card from the top of your library on the outcome of the game. There is a reason that Contract, and other cards with the "Ante" keyword, have never seen reprint.
Abyssal Persecutor is a 6/6 demon with flying and trample that costs only 4 mana. The catch? You cannot win the game and your opponents cannot lose as long as he's in play. However, he can still be quite useful if you have a way to get rid of him after using him to down your opponents' creatures and life totals.
A more recent example would be the aptly named Demonic Pact, which after played, gives you a different positive effect for the next three turns, after which, on the fourth, you automatically lose the game... if your opponent somehow didn't gain control of it, that is.
Lim-Dûl the Necromancer gained a great deal of power and knowledge through deals with the dark planeswalker Leshrac. Ultimately, this turned against him and ultimately made him nothing more than Leshrac's minion and tool.
The Brokers from New Capenna are a demonic law firm who wield a particularly terrifying variation: not only do they make you sign a contract, they erase the memories of said signing, meaning you may very well be one of the sleeper agents whom they will collect/and or enslave without your knowledge. Also they're uniquely white aligned, taking White Magic's association with law to its most disturbing extreme.
One sample monster in Monster of the Week, Uhul, likes to pull this, granting magical power in exchange for souls or other valuable resources.
Monte Cook's World of Darkness: A pact with a demon creates a magic link between the two parties and grants various supernatural enhancements to the recipient. However, demons are spiritual entities from Another Dimension who just use pacts as a ready source of Mana and to psychically manipulate their bonded humans; they just find that people are easier to control when they believe they've signed their souls away.
The supervillain Mr. Infamy is always looking to make this kind of deal. It doesn't matter if it's a normal Joe, a superhero or a supervillain.
In the Freedom City setting, Hades, Lord of the Underworld, has his Soul Pacts. They look good on the surface, but they're depressingly easy to break. The spin-off game Sentinels of Earth-Prime bases its version of Hades around these — when they come into play, the hero that takes them gets great benefits... but if the card is destroyed, the cost comes due.
Played straight and toyed with in Nobilis. The Cammorae play both sides. They're Faust when they make a deal for whatever powers they receive and lose their humanity. They fit in the Mephistopholes role when the player characters contract them to do something. First and second edition Powers of Hell also play with this trope. Many love buying souls, with the full knowledge that such deals don't actually do anything. Humans that think they have no soul have no reason to act virtuously, and will probably become corrupt and hellbound in the process.
The most traditional variation is seen in Demon: The Fallen, in which a Fallen character can make a mortal human their "Thrall". In exchange for the usual stuff like health, luck, romance, etc., the human becomes permanently bound to the Demon and has to fulfill their part of the deal in addition to supplying their master with Faith (used to cast magic). Thralls do have free will and can contemplate a Faustian Rebellion but the only way to break the connection is actually killing their master, which is not exactly simple. Additionally, there are the Earthbound, who Mind Rape their Thralls to make them completely obedient slaves (who cannot provide them with Faith, though).
The Vampire: The Masquerade supplement Guide to the Sabbat had rules for Dark Thaumaturgy, which is Thaumaturgy taught by demons, frequently involving a Faustian pact. Aside from the hatred and fear with which many infernalists (as Dark Thaumaturges were called) were viewed (especially the Sabbat, who are very big on not being under another being's thrall), the rules even specifically stated that the Storyteller now could have the character dragged straight to Hell at any given time, like, say, if they felt the character became TOO powerful.
Mage: The Ascension: This is a regular occurrence for the Nephandi. Finding the most horrific spirit possible and asking it for a job is their standard operating procedure. (Granted, any Nephandus worth the name is fully intending to stab their "patron" in the back and take over as soon as the opening presents itself.)
Pathfinder has plenty of examples, and there's a specific type of devil, the Contract Devil, that specializes in this. In addition, there are several notable examples in the setting itself.
The Thrice-Damned House of Thrune, the ruling family of Cheliax, gained power through a direct deal with Asmodeus during the country's civil war. Despite this, House Thrune does not worship Asmodeus, and often clashes with his faith.
In Curse of the Crimson Throne, Ileosa makes a deal with a belier devil and his master, the exiled Duke Lorthact, binding an Eyrinies to herself in exchange for her soul (though Ileosa was already evil, as a Neutral EvilNay-Theist, she would have gone to Abbaddon, not Hell.) and rulership over Korvosa once she is done with it.
The Witch class makes a pact with an otherworldly and usually menacing patron, who provides them with an enhanced Familiar that acts like a living Spell Book, but similarly to the Warlocknote which was introduced after Pathfinder split off from D&D from Dungeons & Dragons their patron isn't necessarily a fiend.
Princess: The Hopeful: While the Darkness doesn't usually have the intelligence or self-awareness to make deals in the conventional sense, there are various relatively simple rituals which any mortal can use to petition the Darkness for supernatural powers in exchange for inflicting suffering on or killing a sapient being. While the Darkness doesn't explicitly demand your soul in exchange for power, the empowering rituals do leave you Darkened if you weren't already, and the kinds of things you can use Calignes to do will drive you off the Karma Meter in short order.
This is how Spite came Back from the Dead in Sentinels of the Multiverse. After being unceremoniously arrowed in the head by Parse, his spirit made a pact with Gloomweaver, resulting in his "Agent of Gloom" variant. Of course it was a trick. Gloomweaver had rigged Spite so that he could pull a Villain Override and kick Spite's consciousness back into the afterlife when he saw fit, resulting in Skinwalker Gloomweaver.
Binding a demon in Sorcerer (2001) generally involves the sorcerer offering to take care of its Need in return for it granting them its powers. Every player character starts out having already Bound one demon, potentially Binding more as the story develops.
The Chaos Gods of Warhammer and Warhammer 40,000 often empower their mortal followers: Khorne can bestow martial prowess and brute strength, Tzeentch can grant a worshipper sorcerous power or fate-weaving cunning, Slaanesh boosts a person's charisma and makes their senses keener, and Nurgle's followers have their lifespans increased and become significantly tougher. In rare cases, these pacts work out. More commonly, Chaos worshippers fall prey to the very pacts they made: Khorne's followers become mindless killers consumed by bloodlust, spilling their own if no foes present themselves. Tzeentch's devotees end up power-hungry, paranoid, and trapped by the complexities of their scheming, or even betrayed by their patron as part of the god's Gambit Roulette. Slaanesh's disciples become addicted to sensation of any kind, be it mind-rotting drugs or self-mutilation. And Nurgle's cultists are turned into festering, putrid husks, whose corrupt bodies are so tough because they're rotted past the point of feeling pain anymore.
In any case, the road of a champion of Chaos only has two destinations: ascension to a full-fledged daemon prince, or else all the gifts and mutations cause the aspirant's mind and body to collapse into a bestial chaos spawn.
The best part is since these are gods of chaos, making a pact with them in the first place can be difficult - they're just as likely to notice and reward some dabbling nobleman than a warrior who battled for decades in their name.
You can make these pacts in the Dark Heresy RPG, though it may be hard to keep them concealed from your teammates...
A really bad Faust comes from the Warhammer spin-off Mordheim. Nicodemus asked a Greater Daemon of Tzeentch to become "the greatest wizard of the Old World." Wish granted. He discovered an antidote before he grew too large, but he requires a constant supply of Warpstone to manufacture it.
In the Yu-Gi-Oh! game there is a Spell Card called "A Deal With Dark Ruler" where the card art suggests this, showing a man making a deal with Dark Ruler Ha Des, a powerful Fiend-Type Monster in the game. (The card's actual effect has nothing to do with Dark Ruler Ha Des or Fiends; it lets you summon a powerful Nomi monster called Berserk Dragon if one of your a Level 8 or higher monsters is sent to the Graveyard during the turn it is activated.)
: "My summoning begins your debt, planeswalker". The character Liliana Vess
Neutral Evil Nay-Theist, she would have gone to Abbaddon, not Hell.) and rulership over Korvosa once she is done with it.