Those of the Second Kingdom. They wage war with the forces of Heaven. They serve the Demon Emperor Mundus who ruled the Underworld, also known as the Dark Kingdom. 

Mundus: 

Lilith: 

The Destroyer: 

Abigail: 

Argosax: 

Urizen: 

Pluto: 

Abrasix: 

Moloch: 

Belial: 

Nelo Angelo: 

Straga: 

Azmodan: 

Malgos the Defiler: 

Astarte: 

Gollum: 

Maltheal: