I recently finished a game where I clashed with first Jotenheim and then R'lyeh. I mentioned the Jotuns because that delay gave the squishy-boys time to build up their summons. When they attacked, they came led by 3 Vastnesses, backed up by a horde of various other Void Beasts and the obligatory cannon fodder lobo guards and such.
Too bad for them I was playing as Soul Gate Ermor. Those mind bLasts don't bother the mindless much, and even a Vastness gets pulled down pretty fast when surrounded by life-sucking ethereal types.
I love this game. There's a counter-balance for everything! (Did I mention Marignon and their fire-and-brimstone (literally) priest-mages cleaned me out proper very soon after I'd driven R'lyeh back into the depths?)
That was single-player, so the AI
definitely does summon, and very effectively, I might add.