After 500 AD, the Kingdom of Mercia became one of largest and strongest Saxon kingdoms in England, and only faded with the transcendency of the Kingdom of Wessex under Alfred the Great (ruled 871-899).
The first Mercian king to truly dominate England was Penda, ruling from 626-655 AD. Both Bede and Nennius describe the swath he cut across Britain, sometimes in alliance with others (Cadwallon and Cadfael of Gwynedd to name two) and sometime on his own reconnaissance.
His paganism was a particular sore point: “In his Historia Ecclesiastica Gentis Anglorum, written in the early eighth century, Bede of Jarrow describes him as ‘a barbarian more savage than any pagan’ with ‘no respect for the newly established religion of Christ'” and “In the ninth-century Historia Brittonum, Nennius describes Penda as ‘victorious through the arts of the Devil, for he was not baptised, and never believed in God'” http://www.indigogroup.co.uk/edge/penda.htm
According to Nennius, Cadwaladr ap Cadwallon (the hero of my book, The Last Pendragon) regained the throne of Gwynedd from Cadfael, Penda’s ally, and “slew Penda in the field of Gai, and now took place the slaughter of Gai Campi, and the kings of the Britons, who went out with Penda on the expedition as far as the city of Judeu, were slain.” It isn’t clear where this battle took place. http://www.fordham.edu/halsall/basis/nennius-full.html
Penda had a large family and two of his sons became King of Mercia after him. The infighting among the rulers of Mercia rivaled that of Wales. The number of untimely deaths between Penda’s death and Offa’s rule defies belief. After Penda, who died in battle, his successors were: killed in battle, murdered, died, abdicated, poisoned, murdered by his bodyguards, and burnt to death. http://en.wikipedia.org/wiki/List_of_monarchs_of_Mercia
Then came Offa, to whom the building of Offa’s Dyke is attributed. “Forming the traditional boundary between England and Wales, this impressive earthwork runs, although not continuously, from the Dee estuary in the north to the river Wye in the south. Constructed by King Offa of Mercia (757-96), late in the eighth century, it is a tribute to the authority he commanded from the Humber to the Channel. Offa was the most powerful and successful of all the Mercian kings. He dominated England, and his power was acknowledged on the Continent by the great Charlemagne himself. Offa had led many expeditions into Wales, but in his later years he decided upon a policy of stabilizing or at least permanently marking the frontier.” http://www.castlewales.com/offa.html