Coil is a Demiplane inhabited primarily by yuan-ti. It's a dark place with oily soil and snakes that flow like rivers and grow from the ground like twisted mockeries of plants, though actual plants exist there as well. Gray clouds fill the sky, and the whole landscape seems to be surrounded by the coils of a cosmic serpent.
Coil is ruled by Traagash Daur, the Serpent Mother, who dwells in a stone palace of undulating towers and passages.
Source: Ghostwalk, page 120, 145, and 146 (map)