Sigebert I (sĭgˈəbərt) [key], d. 575, Frankish king of Austrasia (561–75), son of Clotaire I. He constantly feuded with his brother Chilperic I, who had inherited the western portion of the Frankish lands, which came to be known as Neustria. The hatred between the two was intensified after Chilperic murdered (567) his wife, Galswintha, who was the sister of Sigebert's wife, Brunhilda. When Chilperic attacked Austrasia in 573, a desire for vengeance made Sigebert vindictive, and in the fighting he overran Neustria. He was about to be proclaimed king of Neustria when he was assassinated by order of Fredegunde, whom Chilperic had married after Galswintha's death.
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