The only direct answer to my knowledge is the 3e fiend of corruption prestige class
The 2003 Fiend Folio published for the third edition of Dungeons & Dragons1 included a “prestige class,” fiend of corruption. Prestige classes were special classes that you could not start in, but were forced to multiclass into after you had some levels in another class and had managed to meet the prestige class’s requirements.2 It was only available to fiends,3 because in third edition, DMs could give class levels to monsters as a way of powering them up. Since Fiend Folio was primarily a compendium of monsters, advertised to DMs, it is likely that the prestige class was not intended to be used by players at all, though players could qualify and play as one under the rules.
Aside from being a fiend, it required charm person or charm monster, 10 ranks in each of Bluff, Disguise, and Sense Motive,4 and a base Will save of +7.5 In 5e, this is roughly equivalent to a requirement that you be 10th level, and have proficiency in Wisdom saving throws, the Deception and Insight skills, and the Disguise Kit.
Anyway, point is, fiends of corruption did just that, they corrupted people. They gain the ability to implant suggestions and even geases in their victims, and can offer bonuses on saving throws, useful fiendish grafts, material wealth (per major creation), or even a wish. In exchange, the fiend of corruption gets the mortal’s soul upon their death—which a fiend of corruption was free to hasten if they wished. The precise use and value of a soul is not defined in Fiend Folio, but it is in a related publication, Book of Vile Darkness (2002). In that book, uses explicitly mentioned include powering infernal machines, as components to evil spells, or to create magic items.
Ultimately, the fiend of corruption class didn’t really make much sense for a PC. Most of the supposed benefits of gaining a mortal’s soul were nebulous and undefined, largely up to the DM—only a few were really spelled out in the rules. Moreover, the timescales involved in corrupting a mortal are really too long for most adventurers.
The only way for a PC fiend of corruption to work was basically if it didn’t bother doing much corrupting, and instead offered the gifts it should be using to tempt people to the party for free. Then it could be a fairly powerful support character (granting their allies various quite-significant bonuses, most notably the once-per-day wish, though the other benefits were not minor). But this didn’t empower the fiend of corruption in any way, and unless the other PCs agreed to the soul bargain, it didn’t even really have much of a flavor reward for the fiend of corruption. Depending on how much good the party is doing, offering such benefits without demanding souls as a reward could very well redeem the fiend (though its powers would still work fine), since there is nothing particularly evil about bonuses to saving throws or conjuring items. So it doesn’t really “work” for the character you have in mind.
But shortly before the “3.5e revised edition,” and containing some-but-not-all updates from that revision—the book is sometimes referred to as “3.25e” as a result.
Unlike 5th edition, non-prestige (“base”) classes in 3rd edition had no requirements for multiclassing.
Strictly speaking, an “Outsider with the evil subtype,” which mostly meant fiends but a few non-fiends could, in fact, qualify.
In 3e, skill proficiency wasn’t an “either-or” thing, but instead every level you got points you could spend to gain ranks in skills. There was also a maximum rank based on your level, such you could not get 10 ranks until 7th level.
3e’s saving throws were Fortitude, Reflex, and Will. Each was calculated as a “base” amount based on class levels, plus an ability score—Constitution for Fortitude, Dexterity for Reflex, and Wisdom for Will. Classes had either “good” or “poor” progressions in each saving throw, so the number of class levels needed for a +7 base Will save depended on what classes those levels were in. For a single-classed character with good Will saves, it required 10 levels. For a multiclassed character with a bunch of different classes that had good Will saves, it could be done sooner, as early as 3rd level.
Better answers would probably come from refluffing
“Refluffing” refers to taking the rules for some option—feat, class, whatever—and using them under a new name, with a different description. This has an advantage, over just creating something wholly new, in that the rules are already tested and presumably found to be functional and not game-breaking. Different editions of D&D have encouraged this more, or less, than others, but it’s a pretty popular thing to do in many of them.
For instance, 3rd edition also had a prestige class called thrallherd, which was a telepath able to manipulate the minds of others and get them to do his bidding. Having this instead be a character offering his thralls and believers a boon of some kind. The thralls and believers come with class levels and abilities of their own—we can just describe those levels and abilities as coming from the thrallherd’s pacts rather than the thrallherd’s telepathy attracting people who already had the appropriate skills.
Unfortunately,6 5th edition does not have the thrallherd option, nor does it have the Leadership feat that the thrallherd’s mechanics are based on. Still, we can just refluff a little harder.
In this case, I’m imagining a warlock, with the Fiend patron. What if, instead of having made a deal with some fiend, this warlock instead was the fiend—and all the people they’ve made deals with, collectively, are their “patron.” That is, I am proposing literally a reverse warlock. As you gain levels, you corrupt and make pacts with more people, and all those souls you’re due empower you, grant you access to still greater fiendish magics. Doesn’t that sound perfect?
And you don’t need to change a thing about the rules for your character, you just change how they’re described. Not every DM will go along with it, but many will. It might be a little awkward if there’s another Fiend-pact warlock in the party—why do you have the same powers when you are doing literally the opposite things?—but you can either focus on making different choices for invocations and spells to differentiate each other, or just explain it away, or just choose some other patron to refluff as the “collected souls” patron. If you have a whole party of warlocks covering all the patrons, then maybe you get yourself in trouble, but meh.
The one thing this doesn’t cover is what, exactly, those you have made deals with are getting. For the most part, I’d say this should be left up to the DM—you helped them with their careers, their love lives, their wealth, whatever. Whatever you gave them, though, it’s theirs, and they don’t want to see you again until it comes time to collect their souls after they die. That’s the easiest way to handle it, keeping it behind the scenes like that. A more ambitious DM could come up with something more involved, though.
- “Unfortunate” only for the purposes of this question; in reality it’s quite fortunate, because thrallherd and the Leadership feat it was based on were quite game-breaking in 3rd edition.