Eki'khas is an extension of the surface lake Vul Lom. Like its surface part, it is a dark liquid that is alien to this world. It was brought here in the Demon Spawn War when a demon lord ally of Demogorgon opened a rift to a watery world of the Abyss. It was intended to be an invasion route, but Orcus was able to buy-off Demogorgon's ally and the rift was abruptly closed.
The liquid from the Abyssal world seeped into Leret Dovah, filling a surface depression and then burrowing into the Underdark where it crept along passages and into caverns without regard to gravity. It is heavier than water, breathable, yet follows no laws of filling the lowest area. In areas of the Underdark, where water does not sit atop it and press it down, it is a sight to see with streams and rivers of the stuff flowing through the air like bridges with fish, crabs, and even the occasional kuo-toa swimming along them.
- Hades, from the Bone Codex - "Demon Waters of Adunamar"