There are these accolytes for them that look like humanoid owls. I don't remember where they all are, but I know there's one in the Water and Fire temples, and one in the cave connecting Yulyana to Florem. When you talk to them, they'll test you to see if you're "worthy" of the summon, and what happens is they start a fight, get a first strike, and use the summon. If one person survives you get it