When the gods died and reality fractured, thousands of planar microportals opened across Odekor. Most of these breaches sealed almost immediately. Some, however, lingered.
One of the largest and the worst occurred in the Underdark. The planar breach was large and opened onto the Elemental Plane of Water. Before the breach sealed, billions of tonnes of water had poured through it, flooding the Underdark.
Untold numbers died in the subterranean flood. Those who could, fled to high points in the shallowest portions of the Underdark. Eventually, even the Underdark’s shallows flooded, and these holdouts were forced to abandon the subterranean world for the surface.
The planar breach responsible for the flood has sealed, but the subterranean world remains flooded. Now known as the Undersea, it is a mysterious and alien place, but hardly lifeless.
Abolethic mermaids haunt the lightless depths of the Undersea, nesting in the drowned cities of the drow and duergar. Water elementals prowl the flooded caverns and tunnels. Swarms of quipper catch the unwary by surprise. Kuo-Toa now make their homes in the deepest depths, far from any light, while powerful marids establish magical fortresses illuminated by shoals of luminous jellyfish.