The Battle of Nechtansmere took place in 685 AD in what is now Scotland. It was a significant conflict between the forces of the Picts, a group of Celtic tribes, and the invading army of Northumbria, led by King Ecgfrith. The battle was fought near a lake called Nechtansmere, and it marked a crucial moment in the struggle for power in the region. The Picts, under their leader Bridei mac Bili, achieved a decisive victory, which halted Northumbrian expansion into Scotland. This battle is often seen as a turning point that helped to preserve Pictish independence and influence in the area for years to come.