The earliest reference to a Pict is that of Calgacus who was defeated by Agricola at the Battle of Mons Graupius. However, Pictish history is increasingly documented in the Early Middle Ages. At that time, the Gaels of Dál Riata controlled what is now Argyll, as part of a kingdom straddling the sea between Britain and Ireland. The Angles of Bernicia, which merged with Deira to form Northumbria, overwhelmed the adjacent British kingdoms, and for much of the 7th century Northumbria was the most powerful kingdom in Britain. The Picts were probably tributary to Northumbria until the reign of Bridei mac Beli, when, in 685, the Anglians suffered a defeat at the Battle of Dun Nechtain that halted their northward expansion. The Northumbrians continued to dominate southern Scotland for the remainder of the Pictish period. Dál Riata was subject to the Pictish king Óengus mac Fergusa during his reign (729–761), and though it had its own kings beginning in the 760s, does not appear to have recovered its political independence from the Picts. A later Pictish king, Caustantín mac Fergusa (793–820), placed his son Domnall on the throne of Dál Riata (811–835). Pictish attempts to achieve a similar dominance over the Britons of Alt Clut (Dumbarton) were not successful. The Viking Age brought great changes in Britain and Ireland, no less in Scotland than elsewhere, with the Vikings conquering and settling the islands and various mainland areas, including Caithness, Sutherland and Galloway. In the middle of the 9th century Ketil Flatnose is said to have founded the Kingdom of the Isles, governing many of these territories, and by the end of that century the Vikings had destroyed the Kingdom of Northumbria, greatly weakened the Kingdom of Strathclyde, and founded the Kingdom of York. In a major battle in 839, the Vikings killed the King of Fortriu, Eógan mac Óengusa, the King of Dál Riata Áed mac Boanta, and many others. In the aftermath, in the 840s, Cínaed mac Ailpín (Kenneth MacAlpin) became king of the Picts. During the reign of Cínaed's grandson, Caustantín mac Áeda (900–943), outsiders began to refer to the region as the Kingdom of Alba rather than the Kingdom of the Picts, but it is not known whether this was because a new kingdom was established or Alba was simply a closer approximation of the Pictish name for the Picts. However, though the Pictish language did not