Attestations
She is first mentioned without name as the beautiful Saxon daughter of Hengist in the Latin Historia Brittonum. Following the arrival of Hengest and Horsa at Ynys Ruym, now known as Thanet, Hengest negotiated with the British High King Vortigern for more land. At her father's orders Rowena gets Vortigern drunk at a feast, and he is so enchanted by her that he agrees to give Hengist whatever he wants if he allowed her to marry him as his second wife. (The fate of Vortigern’s first wife Sevira is unclear). The text makes clear this desire for a pagan woman is a prompting by the Devil. Hengest demands the Kingdom of Kent, which Vortigern foolishly grants him. This agreement is a disaster for the Britons and allows the Saxons to strengthen their foothold in Britain considerably.
Geoffrey of Monmouth's work Historia Regum Britanniae ("The History of the Kings of Britain"), ca. 1138, was the first to give Hengest's daughter a name: Rowena, though the spelling varies widely. In Geoffrey, Vortigern usurps the throne of Britain from the rightful king Constans. Geoffrey claims the drunken seduction of Vortigern created the tradition of toasting in Britain. Vortigern's friendly dealings with the Saxons, especially his allowing even more settlers to join them, causes his sons by his first wife to rebel. His eldest son Vortimer takes the British throne and drives out the Saxons, but he is poisoned by Rowena, who assumes a wicked stepmother role. Later the Saxons kill all the British leaders at the Night of Long Knives, sparing Vortigern because of Rowena. According to the Historia Brittonum, Vortigern "and his wives" were burned alive by heavenly fire in the fortress of Craig Gwrtheyrn ("Vortigern's Rock"), "in north Wales", but Rowena/Rhonwen is not named directly in the passage.
In the Welsh Triads and medieval Welsh poetry, Rhonwen, "the Mother of the English Nation" is an everlasting example of Saxon treachery and paganism. Geoffrey's Rowena with her use of seduction and potions probably forms a basis for villainesses in later Arthurian legend like Morgan le Fay and can be compared with his portrayal of good British Queens like Cordelia and Marcia. There are also connections with the story of Queen Gwendolen and Estrildis, another beautiful German princess. She is also a character in William Henry Ireland's play Vortigern and Rowena.
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