The Grand Corrupter and entourage went hostile and invulnerable on me after a bit of splashback from an AoE attack. Given how they're apparently more than happy to hit each other (at least when they're all aiming at me), it does seem a bit counter intuitive. For the invulnerable part, while they would take normal amounts of damage their HP would not be reduced. I haven't tested to see if this was replicable yet.
The Chronolith twin/clones ability to know where you are constantly, combine with their inability to move within X squares of you allow you to constantly bombard them in complete safety by chucking a large AoE effect around a corner (Corrosive Vapor was what I noticed this with, perhaps other effects are too small? It was just barely large enough). Here's a diagram to further clarify what I mean.
I'm unsure of any other setups in that area offhand which will duplicate that effect, or if any other bosses also try to keep a distance away from the player.
