Initially he was a human Pariah, experimented upon and augmented by Clade Culexus in an attempt to create a potent and more powerful form of Culexus assassin. Whether it was these augmentations or his original nature that made him a Black Pariah is unknown; whatever the case, he was deemed unstable and dangerous by his masters, and they accordingly placed him on a Sisters of Silence vessel bound for the heart of a sun. This vessel was intercepted by a rebel ship carrying Erebus of the Word Bearers Legion. Erebus and his forces boarded the ship and, with the exception of Spear, killed all aboard. Sensing Spear's usefulness, Erebus forced Spear to endure a Chaotic ritual, during which a minor daemon of the warp was bonded to the latter. The bonding of the two produced a highly dangerous predator, one capable of turning a target's psionic force directly back upon it. This ability did, however, require Spear to first obtain a sample of a target's blood, as this was necessary in order for him to gain the synchrony with his target's psionic abilities that allowed Spear to reflect those back at his target.
The Mission to kill the Emperor
Two years after the events of Isstvan V, Spear was tasked by Erebus (seemingly without Horus's knowledge) to assassinate the Emperor. Spear spent a long period of time working towards his goal, access to a document imprinted with a drop of The Emperor's blood. Though Spear eventually obtained the document in question on Dagonet, this act brought him into direct conflict with an Imperial Execution Force--one sent to kill Horus on the same planet--and he was subsequently obliterated.
Horus later admonished Erebus for the latter's plan, declaring that only he--Horus--would kill the Emperor, and no other.