Ry'leh was a desolate planet surrounded by a frozen atmospheric shell. A being claiming to be Azathoth was imprisoned there in the city of Kadath in the Cold Wastes. A dimensional gateway could be opened on The Plain of Leng connecting the planet to Earth. (PROSE: All-Consuming Fire)
Behind the scenes
R'lyeh is the underwater city in the vaults of which Cthulhu, one of the few earthbound Great Old Ones, sleeps. The city is referred to in some of H. P. Lovecraft's work.