Cthulhu

Abyssum is basically the monster of monsters. It can be found in close proximity to the Dark Moon at the depths of Hell.

The boss can shoot singularities at the player and exert a rotating plasma cross of golden lasers.

The entity itself is a Void abomination and the eye of the storm, as if you get too far away, singularities start to be produced everywhere!

This boss is in the background and can move in a peculiar manner

This boss drops a lot of good loot, namely tower wands, 3 perks, full heal and a portal back to the starting mountain.