adding levels or advancing a monster help

[Degwerks]

Out of curiosity, how would you handle a player who had True Polymorph permanently cast on him?

Examples: a level 12 pc turns into an Arcanaloth (cr12), continues to adventure with the party. How would you level that up? Would he gain spells & slots like a wizard would? What about a Deva or Death Knight, if their levels met CR requirements of course.

Some of the creatures mentioned cast spells and prepare them like normal spellcasters and I think they would continue to follow a casting progression of sorts.

I know a player who'd love to have his oathbreaker pally become a Death Knight, thats only possible at lvl 17 though, curious to how he could "level" the remaining 3 lvls with rest of party to 20.

[Shaofoo]

I would do nothing, he will never get any better than what he is now. He can feel free to revert back to his old form if he can find someone who can turn him back and level up from there. I think that should be part of the cost of turning into something else. Or if I was being generous then I'd probably increase HP and add a prof bonus to everything starting from the CR and going from there but no new spells or abilities at all.

[Degwerks]

Yeah I agree the DK probably shouldn't get any new spells or abilities. I missed the part in their description that said they were 19th level casters already.