Tytherion

Tetherion, also known as the Endless Night, was a plane in the Astral Sea in the World Axis cosmology that was home to the dark gods Tiamat and Zehir, the gods of chromatic dragons and serpents, respectively.

No light may pierce its depths, and both dragons and snakes swim through the darkness. It was rumored that the Dragon Queen's hoard rested near the bottom, guarded by her highest servants and favorite children, the ancient dragons.