Obtenebration

Shadow Play (Rank 1)
Gain increased dodge -- do increased damage in dark areas

Shroud of Night (Rank 2)
Blanket a room in darkness

Arms of the Abyss (Rank 3)
Summon tentacles of shadow to assist you

Black Metamorphosis (Rank 4)
Encase your body in an armor of shadow with tentacles growing from your sides

Shadowstep (Rank 5)
Enter the Shadow Plane by making yourself a shadow.