Hell, Devils and Demons

The Devils and demons have been banished to a special plane, Hell, which is effectively a prison with 13 cells each holding a great evil. 
Great power, sacrifice is needed to open a portal to this world and keep it open to allow these beings to come back
These beings are upset and want a way back, and ways are opened by followers and fanatics as they find ancient temples and treasures and uncover pieces of knowledge.
Moloch, Lloth, Orcus and Tiamat are 4 such beings.
Each still has followers and denizens on earth working to break the boundaries and bring there masters back.