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Morrigan was a famous Irish pure-blood witch, notable for being an Animagus who could turn into a crow.
Morrigan was an ancestor of William Sayre, who nicknamed his daughter, Isolt, "Morrigan" due to her affinity for all things natural when she was young.
Transfiguration: Morrigan was so skilled in Self-Transfiguration, that she became an Animagus.
Behind the scenes
According to Irish mythology, the Morrígan ("phantom queen") or Mórrígan (moe-ree-gan) ("great queen") was a figure associated with fate, especially with foretelling doom and death in battle — in this capacity, she would take the shape of a crow, flying above the battlefield.