The Arx Raid
Arx is near the Eye of Terror and watches over one of the smaller routes out of the Eye. Whilst only a monitoring station, Arx was garrisoned with a skeleton Imperial Guard crew to protect the small number of tech-adepts who ran the station.
In 139.M41 a signal was received by the Imperial scout ship Ascendance from Arx's astropath, requesting assistance. When reinforcements (including the 122nd Borlian under Captain Thetis) arrived, they found that the base personnel had been wiped out but the attack had left no traces, other than the grisly and mutilated bodies of their victims.
Upon hearing of the attack, the Inquisition sent Inquisitor Horst to uncover any evidence, but there was little to find. Over the coming three years however, there would be a growing number of similar incidents in the surrounding systems and sectors. These all pointed towards a Chaos attack, which was fulfilled when Abaddon launched his 12th Black Crusade.