Perelandra: A Novel

"Philologist Elwin Ransom, some years after his return from Mars at the end of Out of the Silent Planet, receives a new mission from the Oyarsa (the angelic ruler) of Mars. Ransom summons narrator-Lewis to his country home. Ransom explains to Lewis that he (Ransom) is to travel to Perelandra (Venus), where he is to counter some kind of attack - details not yet specified - to be launched by Earth's Black Archon (Satan)." -- Wikipedia