After reaching the Lower City, Violet, Yousen, Aurelia and Leon will visit camp at night and attempt to kidnap Astarion. She and her fellow spawn can be persuaded that Astarion will help them, but regardless they will be compelled to attack. Failing to defeat them before Astarion reaches 1HP will allow them to take him back to Cazador's Palace.
Violet and the other spawns will later be encountered in Cazador's Dungeon, trapped by ritual ruins as Cazador prepares for his ascension.