It is not separate quest -> you can (it is miss-able) get her summon power during Over Troubled Water (Varena quest, wiki description is just a stub) when you go into hideout in the Baths.
You can summon her during VarenaQuest (Over Troubled Water as it appears in-game), during the baths hideout phase. There's a door with a seal you can obtain after defeating the boss, or you can brute force it if you've learnt Abjure, and inside is a ritual circle you can complete if you collect a few items from the same dungeon (one while exploring, i don't remember the exact spot, but you'll get the prompt to investigate a shelf or something, and the other also by beating the bosses of the dungeon). You'll also need to meet some specific requirements (these ones specificaly), else you get a bad end