Agnes was a hermit nun... And as mad as a hatter. Made it her business to undo curses.
Agnes was a rogue Spanish nun and hermit witch who was obsessed with undoing curses. 700 years ago Agnes locked herself away after having visions of darkness. After decades in exile she would produce the Book of the Damned, a book made from her own flesh and blood, capable of undoing any type of curse. Soon after the book's completion, the Catholic church hierarchy burned her alive at the stake for heresy.
Rowena relates Agnes' story to Charlie Bradbury, explaining how she made it her life's work to learn how to undo curses and how a "cornucopia of curses and satanic visions" led to her being killed by the church hierarchy. Charlie remarks that Agnes was ahead of her time.
While attempting to recruit witches for her new Mega Coven, Rowena tries to entice two witches at a lunch to join her by revealing she is in possession of the Book of the Damned, which is making her a force of nature.
Rowena reveals to Jack that Agnes, in her captivity, watched her friends and only companions starve and perish. She thus created a spell called the Magice Necromantiorum that could bring her deceased friends back to life.