Mairwen is a powerful evil witch until she falls in love with a mortal man named Re Cornelius. However, before she gives birth to Toreth and Taran, she gives Taran to her uncle Dallban and her daughter to her brother, Leopold. She only appears in the Black Cauldron version of Once Upon a Time.
Mairwen is a princess with no interests. For a while, she begins dabbling in the Dark Arts, which her brother Leopold disapproves of. However, after meeting Cornelius, Mairwen turned into a good witch. However, after giving birth to the Twins, she does not tell the Cornelius where their children are at, leading him to become the Horned King. Unknown to him, his daughter was sent to live the life of a princess with Taran being solely raised by their great-uncle.