757. Æthelbald of Mercia killed
Beornred succeeds to Mercia, briefly
Offa succeeds to Mercia
The Anglo-Saxon Chronicle notes that Æthelbald was killed at Seckington and that his body was buried at Repton. It adds that Beornred succeeded to the kingdom but ruled only for a short time and unhappily, and that in the same year Offa came to the throne. Offa's descent is given through Penda's brother Eowa. Simeon of Durham in the 12th century adds the detail that Æthelbald was killed by his own bodyguard.
While earlier kings of Mercia expanded beyond the borders of Mercia proper (e.g. Penda, Wulfhere, Æthelbald), it was under Offa that this expansion reached its greatest extent, involving not just overlordship but direct control over many of the other English kingdoms and marriages with daughters of Offa for the kings of the two kingdoms (Wessex and Northumbria) that lay outside of Mercian rule.
Already in 757 Offa was confirming the charters of the rulers of the neighbouring Hwicce, and by the 790s that ruling family seems to have vanished altogether (see entry on c.670-c.790). Offa may also have had some control of Wessex early on, but Cynewulf seems to have ruled freely for much of his reign (see entry on Cynewulf's accession in 757); the two clashed at Bensington in 779. Offa took control of Kent in 764, lost it again at the battle of Otford in 776, and regained it in 784/5. It was probably shortly after Offa took Kent that he introduced a reformed coinage based on the Frankish model (see entry on c.765); a second coinage reform was made probably in 792. Offa took control of Sussex in about 771, and his control of East Anglia, though it cannot be precisely dated because it is recorded only in the coins, probably dates to the 760s or early 770s (see entry on 749-74). When the East Anglian king Æthelberht tried to declare independence in about 794, Offa had him beheaded.
Offa was married to Cynethryth, who is the only Anglo-Saxon queen to have coins issued in her own name, apparently following the model of the contemporary Byzantine empress Irene (see Grierson and Blackburn, pp.279-80). They had at least three daughters: Eadburh, who married Beorhtric of Wessex in 789, Ælfflæd, who married Æthelred of Northumbria in 792, and Æthelburh, an abbess. The later legends of Æthelberht of East Anglia note that he had hopes of marrying a fourth, Ælfthryth, and an uncertain charter mentions three more daughters (S 127). Only one son is known, Ecgfrith: Offa worked strenuously to ensure that Ecgfrith should succeed him, going so far as to have Ecgfrith consecrated as king while he (Offa) was still alive, following the recent Frankish precedent. It may have been the unwillingness of the archbishop of Canterbury in occupied Kent to oblige Offa on this point which resulted in Offa's scheme to create a third English archbishopric, at Lichfield within Mercia (see entry on 787). Doubtless the papal legates who visited in 786 were involved in negotiations on this point.
Offa enjoyed good relations with the great Frankish king Charlemagne: gifts were exchanged, and in one letter Charlemagne calls Offa "brother" (the only time he uses the term for another western king; see Wormald, p.101). Relations were nearly broken off c.790, probably because Charlemagne was harbouring Offa's political enemies, but were restored later.
Offa's best-known memorial today, the Dyke, leaves no trace in the narrative records, but the continuing battles with the Welsh that accompanied its creation are noted under 760, 778, 784, and 795.
P. Grierson and M. Blackburn, Medieval European Coinage, 1: The Early Middle Ages (5th-10th centuries) (Cambridge: 1986)
P. Wormald in J. Campbell (ed.), The Anglo-Saxons (London: 1982), pp.101-28