unified England, but this is no longer the majority view: in the words of historian Simon Keynes, "Offa was driven by a lust for power, not a vision of English unity; and what he left was a reputation, not a legacy." His son Ecgfrith succeeded him after his death, but reigned for less than five months before Coenwulf of Mercia became king. Background and sources In the first half of the 8th century, the dominant Anglo-Saxon ruler was King Æthelbald of Mercia, who by 731 had become the overlord of all the provinces south of the River Humber. Æthelbald was one of a number of strong Mercian kings who ruled from the mid-7th century to the early 9th, and it was not until the reign of Egbert of Wessex in the 9th century that Mercian power began to wane. The power and prestige that Offa attained made him one of the most significant rulers in Early Medieval Britain, though no contemporary biography of him survives. A key source for the period is the Anglo-Saxon Chronicle, a collection of annals in Old English narrating the history of the Anglo-Saxons. The Chronicle was a West Saxon production, however, and is sometimes thought to be biased in favour of Wessex; hence it may not accurately convey the extent of power achieved by Offa, a Mercian. That power can be seen at work in charters dating from Offa's reign. Charters were documents which granted land to followers or to churchmen and were witnessed by the kings who had the authority to grant the land. A charter might record the names of both a subject king and his overlord on the witness list appended to the grant. Such a witness list can be seen on the Ismere Diploma, for example, where Æthelric, son of king Oshere of the Hwicce, is described as a "subregulus", or subking, of Æthelbald's. The eighth-century monk and chronicler the Venerable Bede wrote a history of the English church called Historia Ecclesiastica Gentis Anglorum; the history only covers events up to 731, but as one of the major sources for Anglo-Saxon history it provides important background information for Offa's reign. Offa's Dyke, most of which was probably built in his reign, is a testimony to the extensive resources Offa had at his command and his ability to organise them. Other surviving sources include a problematic document known as the Tribal Hidage,