Umbral Sun

The Umbral Sun was one of the planet Lubrae's two suns, alongside the Sapphiric Sun.

It is unclear based on Rhulk's transcribed memories whether the Umbral Sun and its Sapphiric counterpart were a naturally-occurring binary pair, or something more exotic; Rhulk mentions that the Witness' Darkness reminded him of the Umbral Sun, suggesting that it may also have been a source of Darkness.