Offa of Mercia ruled much of England from 757 AD to 29 July 796.  http://www.newadvent.org/cathen/11215c.htm 

This date is very exact for that time period and was buried in Bedford.  ”He was succeeded by his son, Ecgfrith, but according to the Anglo-Saxon Chronicle, Ecgfrith died after a reign of only 141 days. A letter written by Alcuin in 797 to a Mercian ealdorman named Osbert makes it apparent that Offa had gone to great lengths to ensure that his son Ecgfrith would succeed him. Alcuin’s opinion is that Ecgfrith “has not died for his own sins; but the vengeance for the blood his father shed to secure the kingdom has reached the son. For you know very well how much blood his father shed to secure the kingdom on his son.”

It is apparent that in addition to Ecgfrith’s consecration in 787, Offa had eliminated dynastic rivals. This seems to have backfired, from the dynastic point of view, as no close male relatives of Offa or Ecgfrith are recorded, and Coenwulf, Ecgfrith’s successor, was only distantly related to Offa’s line.”  http://en.wikipedia.org/wiki/Offa_of_Mercia

Offa’s rule began as a result of violence:  “Æthelbald, who had ruled Mercia since 716, was assassinated in 757. According to a later continuation of Bede’s Historia Ecclesiastica (written anonymously after Bede’s death) the king was “treacherously murdered at night by his own bodyguards”, though the reason why is unrecorded. Æthelbald was initially succeeded by Beornred, about whom little is known. The continuation of Bede comments that Beornred “ruled for a little while, and unhappily”, and adds that “the same year, Offa, having put Beornred to flight, sought to gain the kingdom of the Mercians by bloodshed.” It is possible that Offa did not gain the throne until 758, however, since a charter of 789 describes Offa as being in the thirty-first year of his reign.”  http://en.wikipedia.org/wiki/Offa_of_Mercia

“After he gained power, he consolidated bordering the kingdoms Hwicce and Magonsæte into Mercia. Offa was opportunistic, as when the neighboring kingdom of Kent began to experience some political instability, he enforced himself as overlord of Kent and soon ruled the kingdom of Sussex as well. Offa’s kingdom began to threaten the Welsh kingdoms nearby and Offa soon went to war.

Offa built a series of earthen barriers, or dykes, as fortifications for his units in their war against Wales. It was built in such a way that the Welsh kingdoms would have to charge through a ditch, and then up a hill to gain access to the Mercian soldiers. This put the Welsh soldiers at a severe disadvantage and the Mercians at a tactical advantage. Today, Offa’s Wall makes up some of the border between Wales and England.”  http://yourdailyhistorylesson.tumblr.com/post/685845808/offa-of-mercia

We have no contemporary Mercian source that chronicles his reign.  The Anglo-Saxon Chronicle has been accused of being biased towards the West Saxons, who wrote it, and in light of the future reign of Alfred the Great.   Nennius, although he died during the reign of Offa’s son, only mentions him in the geneologies.  This could be because of Offa’s conflicts with the Church, either over the split in Archbishoprics between Canterbury and Lichfield (at Offa’s request) or the new date of Easter.  http://en.wikisource.org/wiki/Nennius_(DNB00)