Spoiler alert: This post is part of an in-depth discussion of Perelandra by C. S. Lewis, which will inevitably reveal key plot points. You may well have a lot of reading ahead of you.
In a cave deep underground on Perelandra, Ransom defeated the Un-Man, killing the body of his foe Weston and throwing it into an underground fire. After this, he slowly found his way through the dark, subvenerean caverns, seeing sights there that were not meant for humans. Finally, he fell into an underground river, which swept him back to the surface, depositing him in a pool on the side of a mountain. There, he spent some time recovering from his injuries. One remained: a bite from Weston on his heel that would not stop bleeding.
Once he had recovered sufficiently, Ransom descended to the valley below, then ascended the great mountain on the other side. Though reluctant to continue, thinking himself to be approaching a holy place, he felt himself urged on. At the peak of the mountain was a basin or crater, completely covered in red flowers, with a pool at the centre. A coffin like the one that had brought him to Perelandra lay there, open and waiting to return him to Earth. He also perceived two spiritual presences with him.
Ransom recognizes the two beings as one is the Oyarsa of Malacandra (known here only by "Malacandra"; he explains that "Oyarsa" is a title reserved for one's own sphere) and the other is the Oyarsa of of Perelandra. Today is the day that the latter turns the rule of the planet over to the King and Queen. Such had not been possible on Thulcandra (Earth), but here Ransom has made it possible.
The Oyeresu decide it is time to show themselves, and after a few terrifying manifestations, they settle on humanoid figures, thirty feet tall, expressionless and sexless, and glowing white.