When Clotaire I died in 561, his kingdom was divided, in accordance with Frankish custom, among his four sons: Sigebert became king of the northeastern portion, known as Austrasia, with its capital at Rheims, to which he added further territory on the death of his brother, Charibert, in 567 or 568; Charbiert himself had received the kingdom centred on Paris; Guntram received the Kingdom of Burgundy with its capital at Orléans; and the youngest son, the aforementiond Chilperic, received Soissons, which became Neustria when he received his share of Charibert's kingdom. Incursions by the Avars, a fierce nomadic tribe related to the Huns, caused Sigebert to move his capital from Rheims to Metz. He repelled their attacks twice, in 562 and c.568.
About 567, he married Brunhilda, daughter of the Visigothic king Athanagild. This marriage, if we take the chief chronicler of the age, Gregory of Tours, at his word, reveals something about Sigebert's superior character in that violent and lascivious age. As Gregory tells it:
Upon seeing this, Chilperic, the most base of Sigebert's brothers, sent to Athanagild for his other daughter's hand. This daughter, Galswintha, was given him and he abandoned his other wives. However, he soon tired of her and had her murdered in order to marry his mistress Fredegund. Probably spurred by his wife Brunhilda's anger at her sister's murder, Sigebert sought revenge. The two brothers had already been at war, but their hostility now elevated into a long and bitter war that was continued by the descendants of both.
In 573, Sigebert took possession of Poitiers and Touraine, and conquered most of his kingdom. Chilperic then hid in Tournai. But at Sigebert's moment of triumph, when he had just been declared king by Chilperic's subjects at Vitry, he was struck down by two assassins working for Fredegund.
He was succeeded by his son Childebert under the regency of Brunhilda. Brunhilda and Childebert quickly put themselves under the protection of Guntram, who eventually adopted Childebert as his own son and heir.