Fortriu. Their king, Drest mac Donuel, was deposed and was replaced by Bridei mac Bili. By 679, the Northumbrian hegemony seems to have started to fall apart. The Irish annals record a Mercian victory over Ecgfrith at which Ecgfrith's brother, Ælfwine of Deira, was killed. Sieges were recorded at Dunnottar, in the northernmost region of the "Southern Pictish Zone" near Stonehaven in 680, and at Dundurn in Strathearn in 682. The antagonists in these sieges are not recorded, but the most reasonable interpretation is thought to be that Bridei's forces were the assailants. Bridei is also recorded as having "destroyed" the Orkney Islands in 681, at a time when the Northumbrian church was undergoing major religious reform. It had followed the traditions of the Columban church of Iona until the Synod of Whitby in 664 at which it pledged loyalty to the Roman Church. The Northumbrian diocese was divided and a number of new episcopal sees created. One of these was founded at Abercorn on the south coast of the Firth of Forth, and Trumwine was consecrated as Bishop of the Picts. Bridei, who was enthusiastically involved with the church of Iona, is unlikely to have viewed an encroachment of the Northumbrian-sponsored Roman church favourably. The attacks on the Southern Pictish Zone at Dunnottar and Dundurn represented a major threat to Ecgfrith's suzerainty. Ecgfrith was contending with other challenges to his overlordship. In June 684, countering a Gaelic-Briton alliance, he sent his armies, led by Berhtred, son of Beornhæth, to Brega in Ireland. Ecgfrith's force decimated the local population and destroyed many churches, actions which are treated with scorn by Bede. Account of the battle While none of the historical sources explicitly state Ecgfrith's reason for attacking Fortriu in 685, the consensus is that it was to reassert Northumbria's control over the Picts. The most thorough description of the battle is given by Bede in his 8th-century work Historia ecclesiastica gentis Anglorum (The Ecclesiastical History of the English People), but this is still brief. Additional detail is given in the Irish annals of Ulster and Tigernach, and by the early Welsh historian Nennius in his Historia Brittonum (written around a century later). Ecgfrith's attack on Fortriu was made against the counsel of his advisors, including Cuthbert, who had recently been made Bishop of Lindisfarne. The Picts, led by Bridei, feigned retreat and drew Ecgfrith's Northumbrian force into an ambush on