His Assimilation could actually hurt Primordial Civilizations enough that if he devoured significant portions of their authority and connections to Existence, he would actually be capable of stopping their resurrections entirely.
This was a terrifying piece of knowledge to come across as it made absolutely clear exactly why THE Slumbering Genesis and THE Corpse Hierophant would have no issue joining hands despite eternal enmity to coordinate wiping out this common threat. They weren't simply concerned about him being inconvenient or challenging their organizational structures, but rather recognized he represented existential danger to their capacity for eventual return to full power.
