END BOSS at hidden section of 2nd anniversary nikke mini-game

Whats at the end of the hidden section in the nikke mini-game for the second anniversary

[@muiwanderer8798]

This must be Grave. She called us Super human so it must be her!

[@EpicEevees]

WOW YOU ARE A SUPER PLAYER

[@hadesz2257]

Is this bug or what?

[@EpicEevees]

From "Coffin where sin sleeeps - III" if yyou progress down thee hallway there are some challenge rooms leading to this boss