Stryx was a Fallen Angel.
Using the alias 'The Mauler' he took part in the Lemnos Civil War. When subjected to a remote psychic probe by an Angels of Vengeance Librarian, the telltale signs of someone bearing Dark Angels Legion gene-seed appeared. Terminators from the Angels of Vengeance 1st Company were sent in to apprehend him. The strike team was successful in capturing the Mauler, although in doing so, they sustained high casualties. As the Terminators closed upon the surprised mercenary band, there was one amongst their number who shot his way out. At the time, neither the robed stranger, his unusually lethal pistol shots, nor his subsequent escape and disappearance were the known trademark that they have since become. At the time, it was merely noted that the loss of so many Angels of Vengeance Terminators seemed out of place. The veteran warriors in their nigh-impervious armour rarely fell in battle, and when they did, it was to titanic weaponry or some mighty champion, not a rogue soldier from an outlaw band of third-rate mercenaries.
As the Rock was in an adjoining sector, it was not long before the Mauler was secured in its deep dungeons. Persuaded by the harsh methods of the Interrogator-Chaplains and the relentless mind-probes of the Librarians, the Mauler eventually revealed that he had been known by another, older name. Here was Brother Sytrx, a Dark Angel returned to the fold. Before finally recanting and being relieved of the burden of living, Sytrx revealed more. He named a dozen of the Fallen that he had seen in the sector. Elements from the Angels of Vengeance, the Dark Angels and the Lions Sable followed these trails. Each stop along this new journey – from planetfalls to the boarding of a derelict hulk floating in the midst of deep space, led the Unforgiven towards still further clues. At each point, some new revelation was ascertained. As each new scrap of information was reported back to the Rock, a larger and more sinister plot was slowly revealed.