The layer is relatively small and looks like a large bowl canyon. In the center is a large rock. Beams of plasma energy radiate from the rock. The beams are drawn to life and will slay any living being that flies.
About 1,000 years ago Erythnul gave Adimarchus permission to lead an invasion of Celestia. The war went well at first for the demons, but an unknown force (perhaps divine intervention) changes things. Part of Celestia broke off and flew through the gate back to Occipitus. The portion of Celestia embedded in Occipitus and "tainted" the land with a low level of good.
Since Adimarchus's death about 250 years ago other demon princes have tried to invade but have been unable to hold the land. The taint of good may have stopped them.
Travel to Occipitus is difficult; plane_shift does not work. The taint of good thwarts the spell. An item of plane shift built on something from Occipitus will work.