Dark Angels get nasty shock at Paradorn

On 2409.017M42 Titus Luthor's campaign to secure the Foramen Interdictum for the Imperium began. Vital to crossing the Cicatrix, Luthor knew that whoever controlled the end points of the Foramen controlled access through the warpscar, and he was determined to ensure the Imperium held this strategically important pathway. With this in mind Luthor planned to construct two mighty bases, one at Tarlius, the other at Paradorn, but first the Dark Angels sent a probing force to the uninhabited system of Paradorn to establish whether there would be any threat to imperial interests.

There was. Detecting massive psychic emanations from the third rocky planet, the Dark Angels 4th company deployed immediately, only to find - to their horror - a psychic ritual being undertaken by the Thousand sons. Filled with hate the Dark Angels assaulted the traitor legion, disrupting the ritual. It was then that the traitor astarted counter attacked in force, accompanied by none other than the traitor Primarch Magnus the Red and a cabal of Tzeentch daemon princes.

Outnumbered and significantly outgunned, the Dark Angels suffered heavy casualties were are forced to evacuate the few survivors from the expedition. The Dark Angels fleet exited the Interdictum to report what they had discovered to the Inquisition. What was Magnus up to? Why was he in the Aleph Sector?