I think Walpurgisnacht from Madoka Magica should fit the título of TRUE FINAL BOSS for every fictional universe

Walpurgisnacht is the witch in Madoka Magica that every mahou shoujo are destined to fight. why not make it exist in other universes as the ULTIMATE FINAL BOSS, as well as make it 50 times stronger than the strongest villains in every universe it invades. as reveal in the Magia Record game it literally took every single Magical Girl in existence to bring the thing down in the story's perspective. this is always the witch the need to prepare big time before fighting it. this witch when it invades other universes and takes over as FINAL BOSS, instead of seeing cities trashed, you'll see planets, Galaxies, and Etc get trashed.Walpurgisnacht should be the final boss every single fictional character should destined to fight.