Deifr was the Brythonic name for the eastern half of the modern county of Yorkshire

Yorkshire

Yorkshire is a historic county of northern England and the largest in the United Kingdom. Because of its great size in comparison to other English counties, functions have been increasingly undertaken over time by its subdivisions, which have also been subject to periodic reform...

 in northern England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 and was known to the later Anglo-Saxons

Anglo-Saxons

Anglo-Saxon is a term used by historians to designate the Germanic tribes who invaded and settled the south and east of Great Britain beginning in the early 5th century AD, and the period from their creation of the English nation to the Norman conquest. The Anglo-Saxon Era denotes the period of...

 as Deira. Deifr means coastal or waters in Brythonic and was probably a territorial subdivision of the large tribal territory of the Brigantes

Brigantes

The Brigantes were a Celtic tribe who in pre-Roman times controlled the largest section of what would become Northern England, and a significant part of the Midlands. Their kingdom is sometimes called Brigantia, and it was centred in what was later known as Yorkshire...

 before they were incorporated into the Roman Empire

Roman Empire

The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

 in about c.80AD. Its capital was at Ebrauc (modern day York

York

York is a walled city, situated at the confluence of the Rivers Ouse and Foss in North Yorkshire, England. The city has a rich heritage and has provided the backdrop to major political events throughout much of its two millennia of existence...

).

Following the end of Roman rule in Britain

Sub-Roman Britain

Sub-Roman Britain is a term derived from an archaeological label for the material culture of Britain in Late Antiquity: the term "Sub-Roman" was invented to describe the potsherds in sites of the 5th century and the 6th century, initially with an implication of decay of locally-made wares from a...

 the area flourished briefly as an independent realm during the early Dark Ages from the death of Coel Hen in c.420AD (when his kingdom was split between his sons) until c.586 when Ebrauc and much of the rest of the realm fell to the invading Angles

Anglo-Saxons

Anglo-Saxon is a term used by historians to designate the Germanic tribes who invaded and settled the south and east of Great Britain beginning in the early 5th century AD, and the period from their creation of the English nation to the Norman conquest. The Anglo-Saxon Era denotes the period of...

. Once Deifr had been conquered it was renamed in the Anglian tongue "Deira".

Peredur

Peredur

Peredur is the name of a number of men from the boundaries of history and legend in sub-Roman Britain. The most well known of them appear in the following literary and historical sources:-Gwrgi and Peredur, sons of Eliffer:...

 is named as one of the last kings of this realm and scholars identify him with the Sir Percival of Arthurian fame.