He was a really powerful king and was considered the overlord of South Brittain.
Expanded answer:Offa was a powerful king who ruled the dark age Kingdom of Mercia (in central England) from 757AD until 796AD. He is recorded as having annexed to Mercia large areas of the Kingdom of Essex in the 760's, including Middlesex and London. He certainly moved to expand Mercia into the formerly independent lands called Lindsey (northern Lincolnshire) and Megonsæte (Herefordshire) by 765. He invaded the Kingdom of Kent and the Kingdom of Sussex in the 770's deposing their native rulers. He also convinced the Pope to divide the Archdiocese of Canterbury in two by creating a separate Archbishop of Lichfield. In the 790's he conquered the Kingdom of East Anglia and had the king of that realm beheaded in 794.He had now conquered all of south-east England by force. Offa is most remembered for the construction of Offa's Dyke - a giant defensive earthwork fixing the frontier between Welsh and the English.
When Offa died in 796 all of his conquests rebelled against his son Ecgfrith who succeeded him as king. East Anglia, Essex, Sussex and Kent all became independent kingdoms again. After reigning just 141 days his son was murdered by Coenwulf, a rival for the throne.
Offa of Mercia died in 796.
The first British Penny was issued in about 780 AD by King Offa of Mercia.
Offa was king of Mercia. He took control of Hwicce and Sussex, and was overlord of East Anglia. Some claim he was the overlord of all Anglo Saxon kingdoms, but that is not known for certain.
Among other things, Offa built an earthwork wall called Offa's Dyke, which ran nearly the full border between Offa's kingdom of Mercia, and the Welsh kingdom of Powys. This was roughly the border between England and Wales. The purpose of Offa's Dyke is not really known from history, but it can be surmised that it was a defensive barrier and was also a political indication of power and a demarcation of the border. There is a link to an article on Offa's Dyke below.
King Offa of Mercia was responsible for introducing the Penny in about 785 AD, so perhaps he spent the first one.
Offa of Mercia and Edwin of Northumbria started a school to help rulers govern well
Tamworth was the Capital of Mercia. Offa of Mercia is often cited as the first king of England, although it fell back into its regional kingdoms after his death. But I would guess that during his rule Tamworth would have briefly been the capital of England.
The Penny has been referred to as a Penny since they were first Issued by King Offa of Mercia around 780 AD. Prior to that, the predecessor of the Penny was known as a Paeningas or a Dinarii.
There is much discussion about the beginning of English Kings. In Anglo-Saxon times, there were several kings: King Egbert of Wessex and King Offa of Mercia are sometimes described as kings of England, but not by all historians.
Sorry, old bean, but there was never was a King Ortha of England. Could you mean King Offa of Mercia (774-796) whom later (forged) documents described as King of England.
Mercia ended in 918.
Mercia was created in 527.