Wikipedia
Estrildis was the beloved mistress of King Locrinus of the Britons and the mother of his daughter Habren, according to the 12th-century chronicler Geoffrey of Monmouth.
In Geoffrey's pseudohistorical Historia Regum Britanniae (History of the Kings of Britain), Estrildis, the daughter of a king in Germania, was brought to Britain as a captive of Chief Humber the Hun during his invasion following the death of King Brutus. Eventually the invading Nordic "Huns" were defeated by Brutus' three sons, the eldest of whom—Locrinus—fell in love with the beautiful Germanic princess upon discovering her in one of Humber's ships.
Locrinus was forced to honour his prior betrothal to Gwendolen, the daughter of King Corineus of Cornwall, but kept Estrildis as his mistress. For seven years he secretly visited her in a cave beneath Trinovantum (London, i.e., "New Troy"), where she was cared for by servants. Estrildis bore him a daughter, Habren.
When Corineus died, Locrinus deserted Gwendolen and their son Maddan and declared Estrildis his queen. Gwendolen retaliated by raising a Cornish army against Locrinus and defeating him in battle; she then had Estrildis and her daughter, Habren, drowned in a river thereafter called Habren in Welsh (which is the River Severn in English).
Her name is probably a Latinized form of the medieval name Estrild , which survived in England only until the 12th century, according to the 1984 Oxford Dictionary of English Christian Names.