Iago son of Beli was King of Gwynedd, in what is now north-west Wales, in the early seventh and/or late sixth century (the chronology and genealogy of the seventh-century kings of Gwynedd is discussed in another article, Chronology of the Kings of Gwynedd in the seventh century. His death is recorded in Annales Cambriae in the same year as the death of Selyf son of Cynan, King of Powys, at the Battle of Chester (discussed in another article, Dating the Battle of Chester).
This line in Annales Cambriae is often interpreted as meaning that Iago died alongside Selyf at the Battle of Chester. In turn, this interpretation is sometimes further extrapolated as indicating that Powys and Gwynedd, two of the major Brittonic kingdoms at the time, had formed a military alliance against Aethelferth of Bernicia/Northumbria, and/or as indicating that Gwynedd had some territorial interest in Chester. What do we know about the manner of Iago's death?
613 The battle of Caer Legion [Chester]. And there died Selyf son of Cynan. And Iago son of Beli slept [died].
--Annales Cambriae, available online
And that was one of the Three Hatchet-Blows. The second (was) a woodcutter of Aberffraw who struck Golydan with a hatchet, on the head. And the third, one of his own men struck upon Iago, son of Beli, with a hatchet, on the head.
--Red Book of Hergest Triads, available online
Annales Cambriae records the death dates of four out of the seven kings of Gwynedd listed in the genealogies for the seventh century (see article Chronology of the Kings of Gwynedd in the seventh century for details). This may be pure chance, reflecting the material that happened to be available to the scribe compiling the Annales. Alternatively, it may be a deliberate judgement about events that were considered important enough to record, or events that were sufficiently important to act as landmarks distinguishing one year from another. If this is the case, then the fact that Annales Cambriae recorded Iago son of Beli's death may indicate that he was a significant figure.
Did Iago die alongside Selyf son of Cynan, king of Powys, at the Battle of Chester? This is certainly a possibility, since his death is entered in the same year. However, it may be noteworthy that Annales Cambriae treats the two deaths separately. That might be chance or the scribe's personal style, or it may indicate that Iago's death was not directly connected with the battle. Furthermore, the Annales record Iago's death as 'slept', which implies a peaceful death and might be a further indication that Iago did not die in the battle.
In the Harleian manuscript of Annales Cambriae (the oldest version, sometimes called the A text), there are five entries using 'slept' - Latin dormitatio or dormit - in the period from 500 to 750. These are:
--Annales Cambriae in Latin, available online
Ciaran is Ciaran of Clonmacnoise, an important early Irish saint, who died of plague.
Brendan of Birr is another important early Irish saint, roughly contemporary with Ciaran but longer-lived. St Columba apparently saw a vision of angels carrying his soul up to heaven on the night he died, according to Adomnan's Life of Columba (Chapter 11), which sounds like a natural death rather than a death in battle.
Adomnan was abbot of Iona and wrote the Life of St Columba, among other things. As far as I know the manner of death is not recorded, but it seems unlikely that he would have died in battle (if a prominent churchman was killed by violence, one would expect the church to make some outraged comment on it somewhere).
Bede died a natural and peaceful death from illness, as recounted in some detail in an eyewitness account written by Cuthbert, later Abbott of Jarrow.
So these four examples of 'slept' all refer to prominent churchmen. We know two of them died of natural causes, and Bede is explicitly said to have died '...with great devotion and peace'. For the other two, a natural death seems likely though not proven. Deaths explicitly said to be in battle in Annales Cambriae use the Latin 'correrunt' or 'cecidit', usually translated 'fell'. The entry for Selyf at the battle of Chester uses 'cecidit'. Other terms used in the Annales and translated as 'death' include 'moritur' and 'obiit'. So the annalist had other terms to choose from if he didn't want to repeat 'cecidit'; he didn't have to use 'dormitatio' for Iago's death just to avoid repetition. Assuming the choice of 'dormitatio' for Iago's death was intentional and not just whimsy on the annalist's part, it would seem logical that it was intended to be seen as similar to the other 'dormitatio' deaths, i.e. a natural and/or peaceful death. The choice of a word used to describe the deaths of important churchmen may also indicate that Iago himself had entered the church at the time of his death, perhaps having retired to a monastery, although this is speculation on my part.
I have suggested in the article on Chronology of the Kings of Gwynedd in the seventh century that Iago may have been well into his sixties in 613, as this fits with the dates recorded for his predecessors and successors. If Iago was of advanced age, this does not preclude a death in battle if he retained sufficient health and fitness to join the campaign, though it might be considered a slight point in favour of a natural death.
On the other hand, the Triads list Iago's death as one of the Three Hatchet-Blows, implying a violent death, and one of some note. This would be consistent with death in a major battle. A fatal blow from one of his own men could have happened in the confusion of battle (leaving aside sinister conspiracies, out of which a tale might be spun), and is the sort of thing that might have been considered worthy of comment. It seems not to fit well with the natural death that might be implied by Annales Cambriae.
So what to make of this apparent contradiction? Quite possibly, not a lot. Both Annales Cambriae (early ninth century) and the Triads (thirteenth century or so) date from centuries after Iago's time, leaving plenty of time in which they could have been misunderstood or mis-copied. Either or both might be mistaken, or the Annales' choice of 'slept' might be merely idiosyncratic, perhaps a scribe using elegant variation to avoid repeating the same word used for Selyf ap Cynan's death in the same entry.
One possibility is that the Triads may be referring to a different Iago of Gwynedd. Iago son of Idwal son of Meurig* is recorded in the Annals of Ulster as having been killed by his own men in 1039, in one of the various dynastic struggles for rule of Gwynedd:
All these were killed: Iago, king of the Welsh, by his own people; [ .]
--Annals of Ulster, 1039, available online
It may not be beyond possibility that the patronymics of two different Iagos got mixed up at some point, either in copying the Triads or in oral stories and sagas, and that the Iago in the Triads who suffered the hatchet-blow by one of his own men was Iago son of Idwal son of Meurig (1039) instead of Iago son of Beli (613).
It is not unknown for the Triads to incorporate names of figures who certainly do not belong to early medieval Britain. For example, Alan Fyrgan or Fergeant, Duke of Brittany in the late eleventh century, appears in the Three Faithless Warbands Triad. The Triads featuring men and women who received the might of Adam or the beauty of Eve feature Helen of Troy, Paris son of Priam, Hercules and Hector from classical mythology. This does not strike me as surprising; one might expect a living poetic tradition to evolve to accommodate new material, adding the exploits of new heroes as new stories came into circulation.
If the Iago of the Three Hatchet-Blows Triad was a different Iago, perhaps the eleventh-century Iago son of Idwal, then the Iago son of Beli whose death was recorded by Annales Cambriae in 613 could have died a natural death, which may not necessarily have had any connection with the Battle of Chester except a coincidence of year. This would fit with the choice of 'slept' by the annalist, and also with the separation between the mention of the battle and the mention of Iago's death.
As usual, multiple interpretations are possible. It remains a possibility that Iago Gwynedd was killed (by a hatchet wielded by one of his own men) at the Battle of Chester. However, this seems far from proven, and I think the very limited evidence can equally support the less romantic possibility that he simply happened to die in the same year as the battle.
Adomnan, Life of Columba, available online
Annales Cambriae, available online
Annales Cambriae in Latin, available online
Annals of Ulster, available online
Cuthbert, Letter on the illness and death of the venerable Bede, the priest. In: Bede, Ecclesiastical history of the English people. Translated by Leo Sherley-Price. Penguin Classics, 1968, ISBN 0-14-044565-X.
Red Book of Hergest Triads, available online
*A patronymic of a patronymic (a patronymic squared?) is needed here, to
distinguish him from Iago son of Idwal Foel (Iago son of Idwal son of Anarawd)
in the tenth century. See the Wikipedia article on Iago
ap Idwal ap Meurig and Iago
ap Idwal Foel. Given the recurring use of names across centuries, it would
almost be more surprising if individuals didn't occasionally get mixed up.