As others have said, Oathbreaker fits if the player/character's new mission serves the evil power. The alternative could be something else, as whatever force they did serve before is likely to have opinions on what they've just done.
So, these are the questions: Where does the Paladin's power now come from, are they attempting to redeem themselves in the eyes of their old patron, and is this a thing they want to fix over the course of the campaign, a new permenant part of the character spec, or a new direction for the character.
One way to do this would they are now multiclassed, Level X paladin, Level 1 Minion of Lolth, which comes with a couple of really useful spells they wouldn't usually get access to, with the caveat that the more they use these powers the further into debt they become, until Lolth can - for example - claim control of the Paladin for 48 hours, after which they wake up with no clear memory of what they did, their debt paid, and a lot of people with pitchforks outside.
Or they can put XP into climbing your new Minion of Lolth XP tree until the branches start to crack.
Or their current patron doesn't know about the deal, and any of the above options.
But the end point of all of this is up to you and the player to discuss; what tropes they're looking to embody in the campaign, how much this incident becomes/disrupts the campaign, how to balance this rich vein of Content against spotlight time for the rest of the Party.
But if you're looking for templates to add simply as a mechnical thing, Oathbreaker works, Inventing a new multiclass they can maybe only take a level or two of works, something less strictly mechanical that might cause the other characters to start questioning things works (Starting to feel empathy towards demons, suddenly speaking abyssal, a tendancy for flashy spells to now smell of brimstone, that kind of thing).
