The Picts were a confederation of Celtic people who lived in what is now eastern and northern Scotland, which was dubbed as "Pictland". Together with the Gaels, they merged to form the Kingdom of Alba, a precursor to the Kingdom of Scotland.
During the 9th century, the Picts, like the Anglo-Saxons to the south, warred frequently with the invading Vikings from Scandinavia. By the 870s, they warred against Halfdan Ragnarsson, one of the sons of Ragnar Lothbrok, in Northumbria.
During the second century CE, in response to Pictish raids from the north, the Roman Emperor Hadrian built a fortified wall across the northernmost boundary of Britannia to keep the Picts out. For two-hundred years, Hadrian's Wall was effective in stopping the Picts from raiding Rome's holdings in Britain. However, in 410 CE, in the aftermath of the Sack of Rome by the Visigoths, the Western Roman Emperor, Honorius ordered all legions in Britannia withdraw to the continent, thus abandoning Britain. 
This absence of Roman military power gave the Picts the chance to raid south of Hadrian's Wall, pillaging the towns of the Romanized Britons. In 449 CE, a Briton leader named Vortigern realized that his forces alone could not defeat the Picts, and so invited the Anglo-Saxons to fight alongside them as mercenaries against the Picts. This led to the first wars between the Picts and the Anglo-Saxons during the next three centuries, with the latter being unable to conquer their new enemies. 
By the late 6th century, the Gael missionary, Columba, made his way into the Pictish hill-fort at Inverness as a way to achieve peace between the Picts and the Irish kingdom of Dal-Raida. The successful outcome led to the gradual conversion of the Picts to Christianity. 
During the Viking invasions of England in the 9th century, the Picts warred against Halfdan Ragnarsson, the oldest son of Ragnar Lothbrok, all across Northumbria. In 876, the Norse shieldmaiden, Eivor Varinsdottir, helped Halfdan and his army fight against the Picts. 
- Contrary to their portrayal in Assassin's Creed: Valhalla, Pictish armies of the 9th century were armed and equipped with similar weapons and armor to the Anglo-Saxons and the Vikings.
- Similarly, according to stone engravings in Scotland, the Picts also used early types of crossbows from the 6th through 9th centuries, but these were only used for hunting rather than military usage.
- At the time of Assassin's Creed: Valhalla, the Picts were ruled by King Causantin mac Cináeda, son of the first king of Scotland: Kenneth MacAlpin.