She's the specter of Elsa de Longpré, a highly talented actress who was mysteriously killed on stage in 1178 during a performance[1] at the just restored Seidhe Llygad Amphitheater, likely poisoned by a jealous elven actress.[2] True to her words, she in turn haunted the amphitheater, preventing it being used by anyone else.
Upon completing the associated quest on the Nintendo Switch version, cheering and clapping which can be heard during the quest triggers every time the player loads a save created after finishing it.