The Powers of the World

10,000 gods can enter & exit material plane at-will. The power of the gods is linked to the health of the material plane.
Devils can only enter material plane if invited. Devils seek to replace the gods.
Demons can only enter material plan if invited. Demons seek to destroy the gods/ material plane and Devils.
Blood War on lower planes between Devils & Demons
Piety war on higher planes between Devils & gods
Saint's war between gods & demons.