The Historia Brittonum, Chapter 63 begins,
Ada, son of Ida, reigned eight years; Ethelric, son of Adda, reigned four years. Theodoric, son of Ida, reigned seven years. Freothwulf reigned six years. In whose time the kingdom of Kent, by the mission of Gregory, received baptism, Hussa reigned seven years. Against them fought four kings, Urien, and Rydderch Hen, and Gwallawg, and Morcant.
Then continues with the following;
Theodoric fought bravely, together with his sons, against that Urien, . But at that time sometimes the enemy and sometimes our countrymen were defeated, and he shut them up three days and three nights in the island of Metcaut (1); and whilst he was on an expedition he was murdered, at the instance of Morcant, out of envy, because he possessed so much superiority over all the kings in military science.
This is often interpreted as meaning that Urien, the ruler of the Brythonic kingdom of Rheged, led an alliance of native kings in a campaign against the Anglo-Saxons which culminated in a three day siege of Lindisfarne.
A more careful reading of the chapter however, suggests that there is not necessarily any relationship between the first section, which is simply enumerating the sons of Ida, founder of Bernicia and the Brythonic kings who opposed them, and the second which specifically relates to Urien. There is no specific mention of any alliance, and in any event Brythonic kings were as likely to make war with one another as with the Anglo-Saxons.(2)
To the extent that this is a real description of historic events (3), one would have to say that it was only Urien that was involved in the siege. It probably took place sometime in the years between 580 and 590 (4), and the fact that the Bernician defenders were holed up in Lindisfarne indicated how tenuous was their grasp on the kingdom. The assassination of Urien by Morcant (who was probably a prince of the Votadini) is therefore likely to be have been a pivotal event in the history of Bernicia; the failure of the siege to crush the embryonic kingdom allowed its survival and subsequent expansion under the aggressive Aethelfrith.
NOTES
(1) Metcaut, being the original Brythonic name for Lindisfarne
(2) Much the same was true for the Anglo-Saxon kings as well.
(3) And one might doubt it, given that the Historia Brittonum was written, or compiled many centuries after the events it describes; however the account seems believable
(4) Given that the accession of Aethelfrith to the throne of Bernicia can be reasonably accurately dated to 592, the siege must have occurred before this date.