The Picts in the seventh century recognize a single king, Bridei III, who halts the encroachment of the Scots from the kingdom of Dalriada (Argyll).
His claim to the Fortrean Kingship comes through his paternal grandfather, King Nechtan of the Picts.
Nennius' Historia Brittonum tells us that Bridei was King Ecgfrith's fratruelis, i.e., maternal first cousin.
Bridei's mother was probably a daughter of King Edwin of Deira.
Bridei is one of the more expansionary and active of Fortrean monarchs.
He had attacked Dunnottar in 680/681, and had campaigned against the Orcadian sub-kingdom in 682, a campaign so violent that the Annals of Ulster said that the Orkney Islands were "destroyed" by Bridei ("Orcades deletae sunt la Bruide").
It is also recorded that, in the following year, in 683, war broke out between Bridei and the Scots of Dál Riata under Máel Dúin mac Conaill and Bridei's Picts.
The Scots attacked Dundurn in Strathearn.
Dundurn was Bridei's main power base in the south, a great 'nuclear' hilltop fortress.
The Scots apparently did not take Dundurn, and Bridei backed up with an attack on Dunadd, the capital of Dal Riata.
We do not know if Bridei took Dunadd, but the presence of Pictish-style carvings of that time period in Dunadd may mean that he took and occupied Dunadd.
The lack of reputable contemporary sources of this conflict means that not much is known about the Scottish-Pict war of 683, but it is clear that, Bridei, from his base in Fortriu (or Moray), was establishing his overlordship of the lands to the north, and those to the south, perhaps putting himself in a position to attack the Anglian possessions (or overlordship) which existed in the far south.
It is very possible then that Bridei was regarded by Ecgfrith as his sub-king.
The traditional interpretation is that Bridei severed this relationship, causing the invervention of Ecgfrith.
This led in 685 to the famous Battle of Dun Nechtain, in which the Anglo-Saxon army of Ecgfrith was annihilated.
One Irish source reports that Bridei was "fighting for his grandfather's inheritance,” suggesting that either Ecgfrith was challenging Bridei's kingship, or more likely given Bridei's earlier campaigns, that Bridei was seeking to recover the territories ruled by his grandfather in Fife and Circinn, but since taken by the English.
The consequences of this battle were the expulsion of Northumbrians from southern Pictland (established through, for instance, the Anglian "Bishopric of the Picts" at Abercorn) and permanent Fortrean domination of the southern Pictish zone.
