It is quite simple, you need "lingering" debuff / curse from dungeon mobs.
They have around 15% to cast it, type depends on dungeon level.
These debuffs do not disappear after resting and can stack, reducing your stats - but they change some Town content.
Like, you will be able to have fun with Imp maid :) Or group scenes in brothel etc. Or Futa threesome scene with Smith in tavern.