From the World Anvil Summer Camp:
The double sun of the Terciated Land shines upon the Cloudlands and the Underworld alike. Each morning it rises in the east, sets in the west each evening, just like in the world of Weirth. Thus, as the sun makes its way across the sky it crosses the cloud layer twice, first in the morning and then in the evening. When the sun is below the cloud layer it turns a deep orange-red color and casts a haunting, eerie fire-like light across the Underworld. Sunrise and Sunset offer hellish vistas to the people of the Underworld, until the sun passes from sight, either above the clouds or below the horizon. Most of the day the Underworld is lit in a dim, twilight fashion, except for the very rare times the clouds break and a ray of sunshine bursts through.
Read more: The Double Sun