The Planes
The Six Hells
The dimensional domains where Devils and Demons were confined following the Divine Truce. They have been there a very long time. The Truce holds. So far.
Following the Divine War and the subsequent Truce, the beings that had fallen to divine corruption: the first Devils and Demons, were confined to dimensional domains collectively called the Six Hells. The Truce terms prevent their direct entry into the Mortal Sphere. What the Truce does not prevent is influence, cultivation of mortal agents, or the patient work of beings who have had thousands of years to consider their situation and have arrived at various conclusions about how to improve it.
The Six Hells are not a single realm but six distinct dimensional domains, each with its own character and hierarchy. Infernal politics are, by most accounts, significantly more complex than Midralian institutional politics: which is saying something. The Devils and Demons confined there are not uniformly aligned with each other, and the diplomatic situation between the Six Hells and the Mortal Sphere is, depending on which clerical tradition you consult, either carefully managed or quietly deteriorating.