Mordenkainen's Lawbook on Pacts

[Millstone85]

Some of the monster lore in Mordenkainen's Tome of Foes looks like it could be made relevant to a warlock PC. Here are my two cents.

Deathlocks, a Mortgage on Your Soul
A warlock who fails to live up to a bargain with an evil patron runs the risk of rising from the dead as a deathlock, a foul undead driven to serve its otherworldly patron from beyond the grave. (MToF p128)

It is often pointed out on this forum that an otherworldly patron might accept to be paid with something other than the warlock's soul, such as someone else's soul or the fulfilling of a quest of significance to the patron. But the deathlock offers another way to involve the warlock's soul in the bargain, as a secured debt should the warlock fail to provide the alternative. If anything, this should be a very flavorful monster to pit against a party that includes a warlock, even if the DM does not actually plan to ever turn that character into one. It comes in three versions, of CR 3, 4 and 8.

Devils, Straight as the Ruby Rod
[Primus] did, however, order Asmodeus to forever carry a mighty artifact, the Ruby Rod, that would guarantee his adherence to law. The artifact, which has remained at Asmodeus's side ever since, grants him and his underlings the right to enter into contracts with mortals for their souls but unleashes an inescapable punishment upon any devil that breaches such a contract. (MToF p10)

First, a digression. Fiends are supposed to be divided along the law-chaos axis, to the point of being locked into the Blood War because of it. One way this is portrayed is that it is every demon lord for himself while archdevils are hierarchized. However, actual loyalty is rare in the Nine Hells, so it just means political backstabbing instead of open war, except when it is open war. I would like devils to be more into idolatry.

Primus too must have had doubts on the lawfulness of devils, because he is now credited as enforcing it through the Ruby Rod, at least when it comes to Faustian bargains. For a DM, this might be a way to make a warlock, specifically of the Fiend, specifically of a devil, feel like they have some weight in this deal. I would count the risk of becoming a deathlock as making it a contract for their soul.

Maruts, Self-Enforcing Agreements
Primus, the leader of the modrons, created maruts and other inevitables to bring order to dealings between planar folk. [...] The Hall of Concordance is an embassy of pure law in Sigil, the City of Doors. In the hall, two parties who agree to mutual terms---and who pay the requisite gold to the Kolyarut, a mechanical engine of absolute jurisprudence---can have their contract chiseled onto a sheet of gold that is placed in the chest of a marut. (MToF p213)

The clockwork god is really influential these days, isn't he? Personally, I love it! Here, Primus, through his ambassador the Kolyarut, offers another way to portray the pact itself as a tangible force: turning it into a large CR 25 construct. For comparison, demon lords and archdevils in the same book range from CR 16 to CR 26. However, that would involve a trip to Sigil right from the character's backstory, which might be too grandiose a start.

[Finlam]

Maruts, Self-Enforcing Agreements
Primus, the leader of the modrons, created maruts and other inevitables to bring order to dealings between planar folk. [...] The Hall of Concordance is an embassy of pure law in Sigil, the City of Doors. In the hall, two parties who agree to mutual terms---and who pay the requisite gold to the Kolyarut, a mechanical engine of absolute jurisprudence---can have their contract chiseled onto a sheet of gold that is placed in the chest of a marut. (MToF p213)

The clockwork god is really influential these days, isn't he? Personally, I love it! Here, Primus, through his ambassador the Kolyarut, offers another way to portray the pact itself as a tangible force: turning it into a large CR 25 construct. For comparison, demon lords and archdevils in the same book range from CR 16 to CR 26. However, that would involve a trip to Sigil right from the character's backstory, which might be too grandiose a start.

That is some cool lore.

I don't see why you couldn't hijack the main idea of carving the deal into a gold plate embedded in a marut, and move it outside of sigil. Maybe certain churches/kingdoms/guilds have figured out to summon a Marut for this exact purpose. Or maybe they don't even summon it: maybe it's a spell that (along with the acceptable offering) places the contract in the "review queue" so that it will be reviewed and most likely placed in the chest of a Marut.

All you need is an organization that can have a powerful enough caster on payroll and business motive to have a custom spell. It could add a lot of flavor to a game world.

[JakOfAllTirades]

I can each aspect of this lore applying to certain warlock patrons. The Pact of the Fiend, obviously. And Celestial Warlocks who make pacts with lawful good patrons. And certain Undying Patrons would no doubt make undead servants of warlocks who fail them.

But would the Great Old Ones sign a physical contract, or even recognize the laws of gods known to mortals? They're beyond all that... or seem to think they are.

The Archfey would probably laugh at such notions. They've got their own set of rules, which they'll enforce even if they're impossible for their mortal servants to comprehend.

And the lore of Hexblades is a muddled mess so I won't speculate on what their patrons would do.