>>96537665
well since one of their foes is located only in the ghoul stars, and the other are basically servile aliens of the eldar, it stands to reason the chapter they're rivaled to and have animosity about is the Death Spectres. The way they intentionally screw over worlds and populations just to get recruits is rather a bad thing in terms of PR. Not to mention they're tasked with patrolling the ghoul stars so if anyone's logically near to the regiment's sector, it's them.