Zarhulash, known to history as Zarhulash the Potentate, was a C'tan. Like the other star gods of his kind, Zarhulash had been enslaved by the Necrons millions of Terran years before the birth of Mankind as part of their rebellion following the War in Heaven.
A shard of Zarhulash the Potentate was used by the ancient Necrons to power one of the eight Warp beacons known as the Pharos on the world of Sotha in the Ultima Segmentum. The Pharos was built of blackstone, and the array of Pharos devices was originally intended by the Necrons to provide a way for them to achieve rapid interstellar communications and FTL travel after their biotransference.
The birth of the Great Rift that bisected the galaxy at the start of the Era Indomitus reactivated the Pharos. This was discovered by the Scythes of the Emperor Chapter, which still guarded the world after its destruction by a Genestealer Cult infestation and Hive Fleet Kraken during the Second Tyrannic War.
Archmagos Dominus Belisarius Cawl led an expedition to Sotha with the aid of the Ultramarines and the remaining Firstborn Space Marines of the Scythes of the Emperor to study and perhaps use the Pharos' technology to close the Great Rift and defeat Chaos.
The expedition came under assault by Genestealer Cultists who had infiltrated the Chapter Serfs of the Scythes of the Emperor. Ultimately all of that Chapter's remaining Firstborn Marines were slain alongside their Chapter Master Thracius in combat with the Genestealer Patriarch whose cult had first called Hive Fleet Kraken to destroy Sotha.
After reaching the Pharos, Cawl discovered that a shard of Zarhulash had provided the power source for the ancient Pharos for millions of Terran years. He forged a deal with the alien star god, in which Zarhulash agreed to destroy the device by removing its heart, creating a miniature singularity.
In exchange, Cawl agreed to remove the shackles of necrodermis that kept the C'tan Shard captive and create a portal with the Pharos to allow him to finally escape from Sotha.
Zarhulash swore revenge on Cawl for this betrayal.
- Belisarius Cawl: The Great Work (Novel) by Guy Haley, Chs. 8, 9, 23, 26