(Misc reference to Nier and Berserk)
Has a -1 belt slot and -1 ring slot drawback
The berserker mode would put a dark effect, red eye and dark mist around the player when activating the ability, that has no timer, but has a cooldown of 2 minutes upon deactivation. When activating the ability, the player gets higher attack speed, knockback resistance and strength, but loses 1 heart per second
As an easter egg, make the Guts greatsword deal bonus damage on this mode (since its not an endgame weapon at all... But I bet a lot of people wanna use it)
