The planet ruled by the Green Lady in C.S. Lewis’s Space Trilogy is Perelandra.
Perelandra is the title and setting of the second novel in the trilogy, published in 1943. In Lewis’s fictional cosmology, Earth is Thulcandra, while Mars is Malacandra and Venus is Perelandra. The novel’s events take place on a version of Venus that is still close to an unfallen paradise.
The story follows Elwin Ransom, the trilogy’s central human protagonist. He travels to Perelandra and becomes involved in a moral and spiritual struggle surrounding the planet’s first inhabitants. The Green Lady is one of those inhabitants, and her choices are central to the plot.
The book is not a simple retelling of a scientific account of Venus. Lewis published it before modern planetary exploration revealed Venus to be extremely hot and hostile. Its planetary names belong to Lewis’s imaginative theological and literary framework, not to astronomical terminology.