Board Thread:Warhammer 40k General Discussion/@comment-6134883-20130423145616/@comment-7186329-20130423163516

Considering that Abaddon managed to get Angron to participate in the First War for Armageddon, and what the results of that were, if such a union between the demon primarchs was to happen, there would be no current force in the Imperium that could stop them. Angron alone probaly killed a great deal of the defence army and it took an entire company of Grey Knights to contain and stop the attack of chaos, and a dozen? or more to stop Angron himself.

luckily the demon primarchs seem to care more about petty grudges against one another than actually destroying the Imperium. Some have even isolated themselves completely. With that in mind it would probably take an event that could harm Chaos directly, such as the bugs attacking the Eye of Terror, for all the demon primarchs to organize and attack the Tyranids.