Shadows of the Damned

Story related. Cannot be missed. Unlocks upon completing Act 4-5.

Your new weapon, The Dentist will prove to be very useful against those Masked Demons. You'll also run into a new enemy, armored demons that burrow themselves underground. Wait for them to come spinning at you then dodge; they'll knock themselves over and expose the core on their chest. Destroy the core, preferably with the Skullfest 9000 to make quick work of them.