The light that shines down on the Bone Altar, the Black Light is the counterpart of the White Light. By reading the inscription etched into the Bone Altar one can convert to the evil Goddess of the Horde of the Summoned (HotS) Kingdom: Libila.
The Black Light is found on both the Chaos and Elevation servers, and is visible as an immense beam of evil black light that shines down from the clouds. Upon reaching the Black Light, an enormous Bone Altar will be present.
See The Bone Altar for more details.