So I looked at the Chaos Lord, and he can only affect models with the Slaves to Darkness keywords, and neither big nor little Archaon has those. Is that the same in other people's app?
But I did notice the Sorcerer Lord has Oracular Vision which can do the same thing, and he has a Daemonic Power spell to buff another model. So I think I'll switch out Might Lord of Khorne for him.
If I don't turn 1 that'll suck, hopefully there will be some decent terrain again to hide behind. My Hellcannons would like that too