I think a big problem is trying to make Slaves to Darkness allegiance work, when it just isn't viable. The allegiance for Grand Alliance: Chaos will get you more, and then you can run literally anything from the grand alliance freely (even legacy units) without needing allies. And if you really want Archaon to work well it would be using a mono-god allegiance. As to answer your question, Nurgle has several command abilities that work well with him; Harbinger of Decay and Glottkin both benefit marked StDs and Archaon himself.
auticus wrote:Archaon is largely trash right now. The only mileage you will get out of him is the mileage from the gaming table to your carrying case.
The only reason to take him is because you like the model and don't care about getting your face rubbed into the table by virtue of taking a bad list unless your opponent is taking an equally terrible list. If your opponent is even taking a halfway competitive army you are going to get mowed over.
I think this is completely untrue.
IMO Archaon is as good as the player he's used by, every
GT I go to I still see at least one player running him.