Frances de la Tour will play[Spoiler (click to open)] 🙏 Mother Hildegarde 🙏
A key figure in the second book Dragonfly In Amber (which season two is based on), Mother Hildegarde is the intelligent and commanding Mother Superior, who runs L’Hopital Des Anges in Paris. A tall, gaunt woman with a deep and powerful voice, Hildegarde comes from an aristocratic, well-connected family.
When Claire (Caitriona Balfe) begins volunteering at L'Hopital des Anges, she encounters and soon earns the respect of Mother Hildegarde. source 2
De la Tour is known for her roles in Hugo, The Book of Eli, Alice in Wonderland and Harry Potter and the Goblet of Fire.
she is well known, check her IMDB