The Battle of Dun Nechtain or Battle of Nechtansmere is fought between the Picts, led by King Bridei Mac Bili, and the Northumbrians, led by King Ecgfrith on May 20, 685.The Northumbrian hegemony over Northern Britain, won by Ecgfrith's predecessors, had begun to disintegrate.

Several of Northumbria's subject nations had rebelled in recent years, leading to a number of large-scale battles against the Picts, Mercians, and Irish, with varied success.

Following sieges on neighboring territories carried out by the Picts, Ecgfrith leads his forces against them, despite advice to the contrary, in an effort to reassert his suzerainty over the Pictish nations.A feigned retreat by the Picts draws the Northumbrians into an ambush at Dun Nechtain near the lake of Linn Garan.

The battle site has long been thought to have been near the present-day village of Dunnichen in Angus.

Recent research, however, has suggested a more northerly location near Dunachton, on the shores of Loch Insh in Badenoch and Strathspey.The battle ends with a decisive Pictish victory which severely weakens Northumbria's power in northern Britain.

Ecgfrith is killed in battle, along with the greater part of his army.

The Pictish victory marks their independence from Northumbria, who will never regain their dominance in the north.