The Plain of Leng was a location on the planet Ry'leh.
"Azathoth" was imprisoned in the city of Kadath in the Cold Wastes on Ry'leh. Sherringford Holmes manipulated Baron Maupertuis into attacking the city as a diversion, so he could free Azathoth and move her to the Plain of Leng. Once there, he would have the gateway to Earth which was located at the Plain opened so Azathoth could go through and convert humanity to become her servants. (PROSE: All-Consuming Fire)
Behind the scenes[]
The Plateau of Leng was a fictional location created by H. P. Lovecraft. It was sometimes described as being in Earth's Dreamlands, or alternatively in the real world in Asia or possibly Antarctica.