Soemil was an early king of Deira, a territory occupying part of what is now Yorkshire (sketch map here). Historia Brittonum lists him in the Deiran genealogy, with the addition of a cryptic note that he "separated Deira from Bernicia", implying some important action or event. What do we know about him?
61. Woden begat Beldeg, Brond begat Siggar, who begat Sibald, who begat Zegulf, who begat Soemil, who first separated Deur from Berneich [Deira from Bernicia.] Soemil begat Sguerthing, who begat Giulglis, who begat Ulfrea, who begat Iffi, who begat Ulli, Edwin
--Historia Brittonum, available online
--Anglian Collection, available online
Ella was the son of Iff, Iff of Usfrey, Usfrey of Wilgis, Wilgis of
Westerfalcon, Westerfalcon of Seafowl, Seafowl of Sebbald,
Sebbald of Sigeat, Sigeat of Swaddy, Swaddy of Seagirt, Seagar of
Waddy, Waddy of Woden, Woden of Frithowulf
--Anglo-Saxon Chronicle (entry for year AD 560), available online
The lists are broadly similar, with variations in spelling, but not identical. Brond and Baldaeg in the Historia Brittonum list are replaced by Uegdaeg in the Anglian Collection genealogy. Suebdaeg and Siggot in the Anglian Collection list are missing from the Historia Brittonum list. Sguerthing in the Historia Brittonum list is replaced by Uuesteralcna in the Anglian Collection list. From Uilgils/Giulgils onwards, the two genealogies agree (with variations in spelling).
The Anglo-Saxon Chronicle genealogy matches the Anglian Collection genealogy, with variations in spelling, except that Soemil and Saefugul in the Anglian Collection list are replaced by Seafowl in the Anglo-Saxon Chronicle.
The first king of Deira who can be dated securely is Aelle, father of Eadwine/Edwin. Bede tells us that Aelle was reigning in Deira when St Augustine arrived as a missionary to Kent in 597 (On the Reckoning of Time, Chapter 66 (4557)). Bede also tells us that Aelle was king in Deira when not-yet-Pope Gregory the Great saw some Deiran slave boys for sale in a Roman market and made his famous pun, "not Angles but angels" (Bede, Ecclesiastical History, Book II Ch. 1). This happened before Gregory was appointed Pope in around 590 AD, but after he returned to Rome from Constantinople in around 585 or 586 AD. In an earlier article 'Aelle of Deira', I suggested that Aelle's reign may have begun around 570 or 575.
Applying the inexact method of counting generations and allowing 25 years per generation, this would place Soemil somewhere around the middle of the fifth century.
The meaning of this intriguing statement is not certain. We can probably be confident that whoever compiled Historia Brittonum, or its source material, thought that this action of Soemil's was sufficiently important to be worth recording. Moreover, it is the only deed listed for any of the kings between Woden and Edwin, which suggests that it was considered very important indeed.
The kingdoms of Deira and Bernicia were combined, separated and recombined several times during the seventh century, as recorded in Bede's Ecclesiastical History. The two kingdoms were separate before Aethelferth of Bernicia annexed Deira; I have argued elsewhere that this probably happened in about 605 (see article 'Dating the annexation of Deira' for the rationale). Aethelferth ruled both kingdoms until his death in battle in 617, after which Eadwine son of Aelle ruled both kingdoms until his death in battle in 633. In 633/634 the two kingdoms were separated, with Deira ruled by Osric (son of Aelle's brother Aelfric) and Bernicia by Eanferth son of Aethelferth; both were killed within a year. From 634 to 642, both kingdoms were united again under Oswald son of Aethelferth. After Oswald's death in battle in 642, his brother Oswy ruled Bernicia and Oswine son of Osric ruled in Deira until Oswy had him murdered in 651 (yes, being a king in early medieval Britain was a dangerous job). If the compiler of Historia Brittonum was familiar with this to-and-fro, Soemil's 'separation of Deur from Berneich' may have been seen as Round One in a long-running dispute.
The founder figure for the dynasty of Bernicia was Ida, whom Bede says began his reign in 547 (Bede, Ecclesiastical History, Book V Ch. 24). At first sight this is a puzzle; if Ida did not found the kingdom of Bernicia until 547, how could Soemil have separated Deira from it a century earlier?
Two possibilities come to mind (besides the prosaic ones that the names or dates are wrong, or that the entry is fictional):
Although Bede is clear that Ida founded the Bernician (later Northumbrian) royal dynasty, that is not necessarily the same thing as founding the kingdom of Bernicia itself. The name Berneich or Bernicia is of uncertain origin and does not appear to be an Old English name. There seems no reason why the kingdom of Bernicia could not have been in existence, perhaps for some time, before Ida came on the scene. Historia Brittonum provides some support for this in another cryptic remark, saying that Ida united the fortress of Dynguayth with Berneich:
Ida, the son of Eoppa, possessed countries on the left-hand side of Britain, i.e. of the Humbrian sea, and reigned twelve years, and united Dynguayth Guarth-Berneich
--Historia Brittonum, ch. 61, available online
Dynguayth refers to the site of modern Bamburgh, which the Anglo-Saxon Chronicle says that Ida fortified by surrounding it first with a hedge and then with a wall (Anglo-Saxon Chronicle, entry for 547). The statement that Ida "united" his fortress with Berneich is consistent with Berneich/Bernicia being a kingdom that already existed at the time. If this is the case, there is no indication of when it was founded, and therefore no reason why a kingdom of that name could not also have been in existence in Soemil's (approximate) period in the mid-fifth century. Such a mid-fifth-century Bernicia would have had to be much larger than the later kingdom of Bernicia, if it controlled the area that later became Deira, but it is quite possible that kingdoms could have varied in size over time as political units consolidated or fragmented.
The second possibility is that Deira in the mid-fifth-century was subject to some other political authority, to which the compiler of Historia Brittonum applied the familiar name Bernicia. Given the date, only a few decades after the Rescript of Honorius telling the inhabitants of Roman Britain to look to their own defences, and the location of Deira, a short distance east of the major Roman fortress of York, a likely candidate for such a political authority would be whatever continued or succeeded the Roman government based at York.
Very little is known about post-Roman York (more about this in a later article). However, if some sort of local or regional government continued in York after the end of official Roman administration, that would be consistent with post-Roman activity observed by archaeology in other Roman cities (see article on Wroxeter) or Roman forts (see article on Birdoswald) in Britain. If Soemil of Deira was subject to a post-Roman ruler based in York and gained independence from his overlord, this might well have been considered worth remembering. By the time Historia Brittonum was compiled in 830 or so, York was under the control of Northumbrian kings of Bernician descent, and in ch. 50 of Historia Brittonum, Ida is described as "the first king in Bernicia, and in Cair Ebrauc (York).". This is unlikely to be literally true since Bede, who was in a position to know a lot about Northumbrian history, doesn't refer to Ida in connection with York, so it may indicate that the compiler of Historia Brittonum considered that anyone who was a king in York would also be a king of Bernicia (as was the case in the early ninth century when he was writing). In this context, a rebellion against a previous authority based in York could be described as having "first separated Deira from Berneich". Indeed, since there are no contemporary records of fifth-century names, any such post-Roman political entity based in York might even have been called Berneich for all we know (although something based on the Roman name Eboracum or its Brittonic equivalent Caer Ebrauc might seem more likely). If it was a polity covering most or all of the territory controlled by the Roman Dux Britanniarum, it would have extended from York to Hadrian's Wall and could have taken a name equally easily from anywhere in the region.
The most famous account of a rebellion by an English leader against a post-Roman British overlord at some time around the middle of the fifth century is of course the tale of Hengest's revolt against his employer Vortigern. Variations of the story are told in Bede, Gildas, the Anglo-Saxon Chronicle and Historia Brittonum. Briefly, Vortigern was a ruler of part or all of Britain after the end of Roman administration, and hired Germanic mercenaries led by Hengest to protect Britain from the Picts in exchange for grants of land and regular pay. Hengest's troops defeated the Picts, demanded more cash and more land (obtaining Kent, Essex and Sussex by treaty from Vortigern), sent for friends and relatives to join them in Britain, then rebelled, murdered many of the British leaders (but not Vortigern), and plundered large areas of Britain in a destructive raid. Bede dates these events to the middle of the fifth century (Book I, Ch. 15; he gives the initial arrival of Hengest as AD 449, but the sequence clearly took place over an extended period).
The coincidence in approximate dates raises the intriguing possibility that Soemil in Deira might also have been a captain of English federate troops, who was hired under similar terms to Hengest and who rebelled at the same time. This is the sort of thing that might well have been remembered in oral tradition, perhaps hazily as time went on, to be written down centuries later as a cryptic note in Historia Brittonum. I need hardly say that this interpretation is speculative.
Speculating further, it may be noteworthy that the genealogies disagree about Soemil's immediate successor. The Anglian Collection and Anglo-Saxon Chronicle give the name of Soemil's successor as Uuesteralcna or Westerfalcon, Historia Brittonum says it was Sguerthing. Furthermore, in all the genealogies there is a change from the succession of S- names up to and including Soemil to a succession of names beginning with a vowel (W = uu). This change happens immediately after Soemil in two of the genealogies, and after his successor in the other, and is consistent with (but does not prove) a change of dynasty, from a family that favoured names beginning with S- to a different family that favoured names beginning with a vowel. Possibly there were different family factions among the Deiran aristocracy in the mid-fifth century, and possibly Soemil's 'separation of Deira from Berneich' ended up having adverse consequences for his faction or family. If the 'separation' turned out to be temporary, perhaps Soemil's faction was displaced by a rival group when it was reversed. Or if the 'separation' involved breaking the terms of a treaty or agreement, perhaps a rival faction disagreed and took the opposing side.
From the single line in Historia Brittonum and the genealogies, we can reasonably infer that Soemil lived some time around the middle of the fifth century, and that he was remembered for gaining independence for Deira from some other political entity. What that entity was, whether Soemil's independence from it was temporary or permanent, how it was achieved, what its consequences were, and whether it had any connection with the more famous and roughly contemporary rebellion of Hengest, are open to question.
Anglian Collection, available online
Anglo-Saxon Chronicle, available online
Bede. Ecclesiastical History of the English People. Translated by Leo Sherley-Price, Penguin Classics, 1968, ISBN 0-14-044565-X
Bede, The Reckoning of Time. Translated by Faith Wallis. Liverpool University Press, 1999, ISBN 0-85323-693-3.
Historia Brittonum, available online