Locrinus, king of Britain, married her, although he had to be forced to do this by her father. Locrinus had captured the beautiful German maiden Estrildis, whom he would much rather have married. For seven years Locrinus kept Estrildis as his mistress, and during that time she bore him a daughter, Habren, while Gwendolen gave him a son and heir, Maddan. It is not known whether Gwendolen knew of the relationship between Estrildis and her husband.
After the death of Corineus, Locrinus desterted Gwendolen and installed Estrildis as his new queen. Gwendolen returned to Cornwall and raised an army and attacked Locrinus. During a battle near the River Stour, Locrinus was killed by an archer. Gwendolen then commanded that Estrildis and Habren be drowned in a river, but in honour of the fact that Habren was Locrinus’ daughter she ordered that the river should thenceforth carry her name. That river became known as the Sabrina to the Romans, and is today known as the River Severn.
Gwendolen reigned as queen for fifteen years before she abdicated in favour of her son Maddan and retired to Cornwall.
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138