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 reveals to Jack that Agnes, in her captivity, watched her friends and only companions starve and perish. She created a spell called the Magice Necromantiorum that could bring her deceased friends back to life.