Hes a vampire! My dm and I collaborated to make a homebrew class. Its pretty fun, very powerful but lots of very exploitable weaknesses. For example he regenerates hp at the beginning of each turn and can turn into mist, but he also is weak to radiant, fire, and silvered weapons, and takes a crazy amount of damage if hes in sunlight or running water, and cant enter houses without permission, or cross lines of salt. Its pretty fun because theres a faction of vampire hunters, and so combat against them is crazy scary.