The Skeksis performing the Ceremony of the Sun...
...rejuvenating in the process.
A daily morning ritual that the Skeksis perform.
They gather in a circle around the Dark Crystal as the suns of Thra rise, sending light streaming through a hole in the roof of The Castle of the Crystal. That light hits the Dark Crystal and sends beams of purple light into the eyes of the Skeksis, rejuvenating them. However, it only slows their inevitable aging process.
The Ceremony of the Sun is the first scene in The Dark Crystal.