Pathfinder Homegrown Setting (Work in progress)
The chaotic abyss
In legends this place is describes as the dark and waste ocean floor beneath the astrals sea. A dark and murky place filled with the unsteady streams of magic from the planes above. Not much is known about this place, and legends are sporadic in describing in. Most of the times it is said to be so chaotic that the mortals mind could not fathom it.
The only thing known about it is that it is the spawning pool of the primordial evils, and where the most powerful of them resides. From here they first learned to corrupt the tendrils of magic the made of the sea above them.
Other legends claim that the primordials hate the old gods, due to them living above them, and forcing them to carve out existence beneath the currents of the astral sea.
No mortals have ever laid eyes upon this plane. Most likely the untold chaotic horrors of the plane would drive anyone insane. Or the ones soul would be consumed by the powers of the mightiest of the primordial.