Underworld Teoryran (Teoryran World Supplement)
From D&D Wiki
The underworlds are dark continents formed from mountains the size of small worlds. It's green and dark sky are endless caves and in its deeps oceans runs upside through the walls. This is the home of Skaven, of the undead and they fight with many other monsters for their territory. Here there are pathways to hell itself and lakes of fire and many many secrets.