This entry is based on information from Pottermore and does not feature in the original seven novels.
Gormlaith Gaunt was a fanatical Irish pure-blood witch from the 17th century and a direct descendant of Salazar Slytherin. As a member of the notorious Gaunt family, she was a staunch believer in pure-blood supremacy and a powerful practitioner of the Dark Arts. Gormlaith serves as the primary antagonist in the origin story of Ilvermorny School of Witchcraft and Wizardry, acting as the cruel aunt and captor of the school's founder, her niece Isolt Sayre. Her persecution of Isolt was the direct catalyst for Isolt's flight to North America and the subsequent founding of Ilvermorny. (Pottermore)
All biographical information for Gormlaith Gaunt comes from the story of the founding of Ilvermorny School of Witchcraft and Wizardry, as published on Pottermore.
Gormlaith Gaunt was born into the Irish branch of the Gaunt family and lived in the valley of Coomloughra, Ireland. She was raised to embrace the family's violent and grandiose belief in pure-blood superiority. Gormlaith had a younger sister, Rionach, who did not share her fanatical views and was known for her kindness toward her Muggle neighbours. When Rionach married a Muggle-born wizard, William Sayre, and gave birth to a daughter, Isolt Sayre, Gormlaith felt her family line had been betrayed and irredeemably tainted. (Pottermore)
In approximately 1608, consumed by rage and a twisted sense of duty, Gormlaith went to her sister's home. She murdered both Rionach and William Sayre and set their cottage on fire. She then “rescued” the five-year-old Isolt from the flames, bringing her to the neighbouring valley of Coomcallee, or “Hag's Glen.” Gormlaith intended to raise Isolt with the belief that her parents' Muggle-loving ways had caused a curse that led to their deaths, ensuring the girl would grow up to follow the pure-blood doctrine. (Pottermore) Gormlaith forbade Isolt from attending Hogwarts School of Witchcraft and Wizardry, claiming it was filled with “Mudbloods and Muggle-lovers.” For twelve years, she kept Isolt completely isolated, using powerful Dark Magic to enforce her captivity. However, Isolt secretly learned to use her own wand by observing her aunt. Eventually, Isolt managed to steal Gormlaith's most prized possession—the wand of their ancestor, Salazar Slytherin—and fled to the New World. (Pottermore)
For years, Gormlaith hunted for her niece. She eventually learned of Isolt's whereabouts in North America after overhearing other families discussing the new school of magic, Ilvermorny. Realising Isolt had married a Muggle, James Steward, and started a family, Gormlaith travelled to Mount Greylock, Massachusetts, bent on revenge. Her goals were to destroy the school, kill Isolt and her husband, and abduct her great-nieces to raise them as “proper” pure-bloods. (Pottermore) Upon arriving at the school, she used Parseltongue to send a command to Salazar Slytherin's Wand, which was kept inside the castle, causing it to become inactive. She then placed a powerful curse on Isolt and James, forcing them into a deep, enchanted sleep. However, her entrance was thwarted by Isolt's adopted sons, Chadwick and Webster Boot, whose wands, containing cores of Horned Serpent horn, vibrated to warn them of Gormlaith's Parseltongue. (Pottermore)
Gormlaith engaged in a fierce duel with the Boot brothers, who proved surprisingly resilient. The duel was interrupted when Isolt's infant twin daughters began to cry, their screams breaking the curse Gormlaith had cast on their mother. Awakened, Isolt called for help from her old friend, William the Pukwudgie. As Gormlaith turned to face Isolt, William appeared on a windowsill and shot her through the heart with a poison-tipped arrow, killing her instantly. Her body turned to dust, leaving only a pile of smoking robes and the broken, blackened remains of her wand. (Pottermore)
Gormlaith's specific physical appearance is not described in detail. (Pottermore) Her personality was defined by her fanatical devotion to pure-blood ideology. She was cruel, violent, and utterly ruthless, willing to murder her own sister and brother-in-law for marrying outside what she considered acceptable blood status. She was arrogant and believed her lineage made her superior to all others. Despite her cruelty, she operated under the delusion that she was acting righteously by “rescuing” Isolt from a life of Muggle tolerance. She was an incredibly powerful but malevolent and hateful individual. (Pottermore)
Gormlaith was a highly accomplished and formidable witch, with a particular talent for the Dark Arts.
Gormlaith Gaunt does not appear or receive any mention in the seven original *Harry Potter* novels. Her entire character history was created by J.K. Rowling for the short story Ilvermorny School of Witchcraft and Wizardry, published on the Pottermore website on 28 June 2016. Her story provides crucial background for the founding of the North American school of magic. (Pottermore)