Idea
The Pun Tyrant
Yep, he's been using that stored XP to fuel some of his spellsAinz to my knowledge has been avoiding xp draining moves, other than his attempt to use Wish Upon A Star, which didn't work so we aren't sure if it cost him, on top of that max level players can build additional xp.(one level worth) In the novel Ainz maxes this reservoir out in an attempt to see if he could level up further. Fun fact he has a world class Item Greed and Generosity that allows xp to be stored and saved within it.