Alcetas Castael was a Blood Angels Archein, during the Great Crusade and Horus Heresy.
As the Great Crusade neared its end, the strike force led by Castael was too far away to join the Blood Angels muster for the Signus Campaign in time. Shortly afterwards, the Horus Heresy began and Castael's forces spent the next five years fighting to survive the horrors unleashed by Horus. In the 5th year of their journey, though, the Archein decided to make for his Homeworld Baal, in order to relive the siege it was under. However before they reached it, the strike force received an Astropathic message stating the nearby Gilden's Star had fallen to Heresy and rebelled against the Imperium. This incensed Castael, as his Legion had brought the System's worlds into Compliance, during the Great Crusade. He resolved to reconquer Gilden's Star, not only to bring retribution for its betrayal, but also to show the Blood Angels were not dead, as had been suspected at that time during the Horus Heresy. The System's resources and proximity, would also give Castael an ideal staging ground to relive the siege that was currently surrounding Baal.
When they arrived there, the Archein led an invasion of the Gilden's Star from aboard the Judicator, which served as his strike force's flagship. His warships then laid waste to the Capital World Gilden Prime's defenses, before Castael deployed with the Blood Angels to its surface. As he could ill afford to be bogged down in a prolonged fight, the Archein proceeded to split his forces so they could seize key locations necessary to conquer the world. However the Word Bearers, of the Dark Apostle Zaxos, had been behind the Gilden's Star's fall to Heresy and they had prepared for the Imperium to attempt to reclaim it. Soon afterwards, the Blood Angels' communication equipment began to be jammed, which prevented Castael's separate groups from contacting each other or their fleet in orbit. Then the Word Bearers launched a series of ambushes, with the corrupted population of Gilden Prime, which overwhelmed the Blood Angels. The Word Bearers' Procurators also began summoning Daemons, by desecrating the bodies of dead or wounded Blood Angels as sacrifices.
Despite fighting ferociously, and dealing the Traitor Legion severe losses, Castael's forces were quickly slaughtered and soon only the Archein and a small number of Blood Angels near him still lived. Just as they were about to be killed, however, a small force of Crimson Paladins appeared from the Blood Angels' fleet in orbit. The Exemplar leading them, stated the Blood Angels' warships had been attacked by a Word Bearers fleet and had already lost several warships to the Traitors. The Judicator's Captain, had sent the Paladins with a request for Castael to return to their fleet to save it or else their warships would be lost. The Archein agreed and took a Teleportation Beacon from the Exemplar and reached his Flagship, shortly before the Blood Angels on Gilden Prime were killed. Despite his efforts, though, Castael's fleet began to overwhelmed as well and he ordered the Dawn's Vigil to escape the System and warn the Imperium about what had occurred there. Its commander refused to do so and instead rammed the Word Bearers' flagship, Shard of Darkness, before firing its Nova Cannon.
The resulting explosion not only destroyed both warships, but also most of the Word Bearers fleet as well. This caused the Traitor Legion's surviving leaders in the System to flee soon afterwards, leaving its Word Bearers and Gilden's Star's population to fend for themselves. The Blood Angels were not merciful to those left behind and began scouring the worlds of the Traitors' presence. Witnessing their Brothers being desecrated by the Procurators had awoken something dark within Castael and his surviving forces, which led them to bring Gilden Star's worlds to ruin. Gilden Secundus, Gilden Quintus felt their vengeance and though, Castael was declared missing, and presumed dead, during the Blood Angels' orbital drop on Gilden Tertius, they continued on without him. By the time the Archein's strike force finally left the System, billions had been slain and word spread of the massacres the Blood Angles had carried out. This severely weakened the Imperium's cause, during the Horus Heresy and the Gilden's Star's neighboring systems wavered in their support for it afterwards. Some of the Systems' worlds even completely withdrew from the Imperium and cut off all contact. It remains a point of contention among Imperial historigraphers, on whether this had been Zaxos' true goal all along. As for Castael's strike force, the losses they suffered in the Campaign left them Beverly weakened and prevented them from launching any major operations against the Traitor forces during the Horus Heresy. Their victory also became a stain on the Blood Angels' history, which greatly shamed the Legion.