Its a new boss with 790,000 hp,its at the Underworld Dimension, you get teleported there by clicking Odd_developer
It drops: Shattered Flame , Saitama's Cape, Glitched Lightning
It appears that Glitched Lightning is most common, followed by shattered flame and Saitama's Cape.
How the boss looks like:

Underworld boss Itself (lol)
Note: This boss was removed.