Cosmology (Zyanya Supplement)
From D&D Wiki
The Celestial Realms, known as heaven, exist above the sky. There are legends of some who have managed to fly high enough to reach those lands, and a few nearly unclimbable mountains that reach those heights.
Below even the Underdark lies the underworld, where the souls of the dead go. Souls that never reach the underworld rise as undead, while souls that manage to escape crawl across the world as aberrations.