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Offa, king of Mercia seized the throne after a civil war, and established supremacy over many lesser kings. He consolidated his position by marrying his daughters to the kings of Wessex and Northumbria, and was the first ruler to be called 'king of the English'.
Offa ruthlessly overcame strong opposition in southern England. By the end of his reign, Offa was master of all England south of the Humber. He had a frontier barrier (Offa's Dyke) built; this continuous ditch and bank ran 149 miles along the boundary between the Mercian and Welsh kingdoms 'from sea to sea'. Offa had dealings with the emperor Charlemagne (a proposed dynastic marriage between their children came to nothing), and he visited Rome in 792 to strengthen his links with the papacy. The English penny (silver currency) was introduced during Offa's reign. In the first recorded coronation in England, Offa's son Ecgfrith was consecrated in 787 in Offa's lifetime in an attempt to secure the succession. However, Ecgfrith died childless, months after Offa. Offa's success in building a strong unified kingdom caused resistance in other kingdoms. The Mercians' defeat at the hands of Egbert of Wessex at the battle of Ellendun in 825 meant that supremacy passed to Wessex.