The Godstalker is an entity empowered by the Celestials to impose their laws in the universe.
Thousands of years ago, the Celestials exiled the Deviant conqueror Lord Tantalus to planet Earth. When Tantalus finally repaired his ship's stardrive and returned home to Armechadon, the Godstalker was dispatched to enforce the Celestials' ruling. However, the Godstalker was working from outdated information and believed Tantalus was incapable of conceiving children. When he arrived on Earth, Godstalker detected Tantalus's unique genetic energy signature, the Black Legacy, in Tantalus's son Lucian. The Godstalker refused to believe Lucian's claims to be Tantalus's offspring, and believed Lucian was merely Tantalus in disguise.
When the Eternal Druig attempted to steal a Celestial weapon to use against his creators, he was atomized in the effort. This injury wasn't fatal (at least to an Eternal), but his essence was collected and detained by the Godstalker in the Desecration Annex for his crime.
Because of over-reliance on his sensors, the Godstalker had difficulty telling Lucian from his father Tantalus, and attacked him by mistake, believing it was impossible for Tantalus to bear a child.