Angel Of Destruction Kamael

Kamael is the first boss from the fallen underground, you access him by approaching the devil teleporter Devil Viraju (the witch doctor with a cyclone on top of him) then select him and press Q to access kamael's area.

He drops a nuke once you approach him.

Drops:

Spirit - Kamael's Spirit

(This spirit is the one you will use to upgrade all the cain gear).

Others: Weapon - Greatsword of Chaos

Grimoire: Consume fear