Umbral Sun

Overview

The Umbral Sun was one of the two suns over Lubrae, alongside the Sapphiric Sun. It left the surface of the planet in darkness. It was first mentioned in Destiny 2: The Witch Queen.

Darkness

Whether the two suns were a natural binary pair or something stranger is not settled. Rhulk compares the Witness's Darkness to the Umbral Sun, which suggests it may also have been a source of Darkness.