Studying the charts of the region around the Warp Gate, Achilus settled on a number of possible life-sustaining worlds that would suit his purposes. Most prominent among them were Karlack, Calisi, Alphos and Spite.
Though Calisi and Alphos were closer to the Warp Gate than either Karlack or Spite, Achilus ended up discounting them, even though they would make excellent outposts and strong points. They were too poor in resources to support the kind of production he wanted to eventually unleash for the crusade.
However, sometime around 777.M41, during the first phase of the crusade, Imperial forces spread out from the Warp Gate and set out for the worlds of not just Calisi and Alphos, but Hethgard, Pyrathas, Spite, and Karlack. Alphos fell within a standard year of the initial invasion, without any severe losses.
With the crusade's second wave, Alphos, alongside the other conquered worlds, experienced rapid fortification from the Imperium, turning what was once a minor colony into a towering edifice of plasteel and ferrocrete bristling with Macrocannon and lance turrets.
In less than five standard years, Achilus made Alphos part of what would become the Iron Collar -- a series of Fortress Worlds surrounding the Well of Night, thereby warding the Warp Gate from invasion.
It was in 786.M41, during a routine voyage from Alphos to Karlack that the lord militant's vessel Proclamation of Wrath was lost to the Warp. Indeed, rumours spread of what may have happened.
The most disturbing of these, however, and the one which haunted the entire investigation for the missing vessel, was the discovery that the Aeldari assassin Syndilian Shanyr had been sighted on Alphos only solar hours after the departure of the lord militant.
- Deathwatch - The Achilus Assault (RPG), pp. 14-17, 21-23