Offa of Mercia was the king of Mercia from 757 until his death in July 796. He was the son of Thingfrith and a descendant of Eowa, a brother of Penda who ruled over a century before. Offa came to the thrown after years of civil war following the assassination of Æthelbald. In the early years of his reign it is likely that Offa consolidated his rule after defeating Beornred to become King. In 762 he became the overlord of the Kingdom of Kent, taking advantage of the kingdoms instability, and was in control of Sussex until 771, although his authority was challenged in both areas during his reign. He extended his power of most of southern England in the 780s, allying himself with Beorhtric of Wessex who married Offa's daughter, Eadburh, and regained complete control of the southeast. He also became the overlord of the Kingdom of East Anglia in 794 and had Æthelberht II of East Anglia beheaded in 794.
Offa was a Christian king, but came into conflict with the Church, and in particular with Jaenberht, the Archbishop of Canterbury. Offa managed to persuade Pope Adrian I to divide the archdiocese of Canterbury in two, creating a new archdiocese of Lichfield. This reduction in the power of Canterbury may have been motivated by Offa's desire to have an archbishop consecrate his son Ecgfrith of Mercia as king, since it is possible Jaenberht refused to perform the ceremony, which took place in 787. Offa had a dispute with the Bishop of Worcester which was settled in the Council of Brentford in 781.
Many historians regard Offa as the most powerful Anglo-Saxon king before Alfred the Great. His dominance never extended to Northumbria, though he did marry a daughter, Ælfflæd, to the Northumbrian king Æthelred I in 792. His reign was once seen by historians as part of a process leading to a unified England, but this is no longer the majority view. In the words of a recent historian: "Offa was driven by a lust for power, not a vision of English unity; and what he left was a reputation, not a legacy." Offa died in 796 and was succeeded by his son, Ecgfrith, who reigned for less than five months before Coenwulf of Mercia became king.