Badab Sector Pirate: void trooper of the 33rd "Marine Hunters"
The imperium has been trying to secure the Badab sector since the great crusade, yet the Emperor himself could not secure the maelstrom and so it was eventually abandoned, this of course means there is likely a large amount of heresy era tech in the maelstrom. when the Badab war ended, all surviving human forces in league with huron joined him in his flight into the maelstrom, I imagine these soldier found a cache of heresy era Auxiliary armor and weapons, pledged their allegiance to Huron and returned to raid the badab sector with their new powerful toys. Only the best of the best survive in these harsh conditions meaning anything short of a space marine is no match for these hardened warrior. - Also they are not chaos worshipers, they are renegades and only seek revenge on the imperium. Naturally these warriors are hold particular hate for marines who they would likely have seen butchering their comrades without remorse so they like to return the favor as often as they can