Ry'leh

 was a desolate planet surrounded by a frozen atmospheric shell. It was where Azathoth was imprisoned. A dimensional gateway could be opened connecting the planet to Earth. (PROSE: All-Consuming Fire)

Behind the scenes
R'lyeh is the underwater city in which vaults Cthulhu, one of the few earthbound Great Old Ones, sleeps. The city is referred to in some of H.P. Lovecraft's work.