Carissa doesn't go to the front lines, she stays back in the fort and works during the days and flirts with adventurers until they let her look at their swords, in the evenings. Someone offered to teach her Fireball just in case, now that she has third-circle spells, but she's saving up for a Ring of Sustenance and then a headband and declined.

This means that she is defenseless when ambushed by a kind of demon she's never seen before that must have bypassed the wards somehow and bypassed the fort's guards somehow and honestly it probably wouldn't have even helped to have Fireball but it would've felt less stupid than standing against the wall and screaming.

And then she's somewhere else. Presumably the Boneyard? Only it's not really as described.