The military prowess of King Onuist son of Vurguist (732– 761) is unparalleled in Pictish history. The Gaelic poem Fó sén dia ngaib Óengus Albain trumpets that it was “good the day” when Onuist claimed Alba (Pictland). He smote the Dál Riata and English chronicles denounced him as a “despotic butcher” whose entire reign was stained “with criminal blood.” With such contrasting and impassioned attitudes towards Onuist's reign it is unfortunate that our sources only sketch an outline of his career. Historians must therefore infer the political trajectory of his reign from chronicle entries of conflict. Onuist's origins are also obscure. A tract from Munster (Éogan Már trá óenmac leis) claimed he descended from Irish Éoganacht settlers in the Mearns of Circin. This is interpreted as either a reference to a contemporary alliance or a later construct by the medieval Scottish administration to prove their Gaelic (Irish) ancestry. Onuist's home territory, the Mearns (A’ Mhaoirne), translates as “a territory under stewardry” (or Maerdom); Dunottar fort lies within this district. While Onuist's dynasty of Vurguist supplants the Derelei line, it retains a connection with Fortriu— either genealogical or as a title of kingship. For although Onuist was rex Pictorum in the records, his brother and successor Bridei was rex Fortrenn. Similarly, Onuist named his sons Bridei and Talorcan, which suggests they were affiliates of the Fortriu dynasty.

Onuist's reign is characterized by shifting political alliances. His early career was concerned with the removal of rival factions, beginning with Talorg of Atholl. A putative alliance between Talorg and the Cenél nGabráin may have motivated Onuist's son Bridei to join with the Cenél Loairn to attack Kintyre in 731. Two years later this alliance was defunct when Dúngal of the Cenél Loairn forcibly removed Bridei from the precincts of the Columban monastery on Tory Island, Ireland, after which Dúngal was removed from leadership by his uncle Muiredach. A year later Talorc of Kintyre was handed over to the Picts by his brother and summarily drowned. Then Talorg of Atholl was taken near Dunollie, bound, and five years later drowned by Onuist. The return of Dúngal to Argyll in 736 appears to have prompted Onuist to personally lead an assault on the region.