William Sayre (d. 1608) was an Irish pure-blood wizard. He was the husband of Rionach Sayre and the father of Isolt Sayre. Isolt was one of the founders of Ilvermorny School of Witchcraft and Wizardry. William showed sympathy towards his Muggle neighbours and because of that was murdered by his sister-in-law Gormlaith Gaunt.
William was a descendant of Morrigan, an Animagus whose creature form was a crow. By 1603, he had married a pure-blood witch, Rionach Gaunt and was living at Ilvermorny Cottage in Coomloughra, County Kerry, Ireland. Their daughter, Isolt, was born that year.
William nicknamed his daughter ‘Morrigan’ for her affinity for all natural things when she was young. He and his wife loved their daughter very much and were quietly helpful to their Muggle neighbours, producing magical cures for humans and livestock alike.
However, when Isolt was five, William and Rionach were killed in a fire started by his estranged sister-in-law, Gormlaith Gaunt, who "saved" Isolt with the intention of raising her with a belief in pure-blood superiority. Isolt eventually fled Gormlaith and moved to the New World, where she befriended a Pukwudgie, who she named William in honour of her father. William had two granddaughters through Isolt's marriage to James Steward, Martha and Rionach Steward, as well as two grandsons by adoption, Chadwick and Webster Boot.