The Picts
The creation of the Picts is largely conjecture. It is simply unknown the full story of how the Picts came to be, though when and why does occur. When the Romans conquered and annexed portions of Britain, many British nobles took their armies, their families, and their loyal retainers, who did likewise, and fled north, into the isolated lands of Caledonia. Some settled in the south, but many went further, ultimately settling in the distant highlands, seemingly without much contestation from the local Caledonian tribes. The Irish record vague dealings with the early Picts, include helping them establish a confederated authority, but the validity of such claims is spotty; all the same, it did not stop such claims even as late as the renaissance. In any event, the Picti came about, and were divided into seven kingdoms that ruled much of what is now Scotland.
According to Pictish legend, there was once a king named Cruithne, son of Cing. Cruithne reigned for one hundred years, and had seven sons, who were named Fibb, Fidach, Fotla, Fortriu, Cat, Ce and Circind. When Cruithne died, his kingdom was split into seven regions, each one given to one of his sons. This is clearly affected by the Gaels; Cruithne is itself a Gaelic word for 'Picts', and is what they referred to both the pre-Gaelic inhabitants of Ireland and Scotland as. However, the seven kingdoms did exist, and were recognized as the seven ruling principalities of the Picts by both themselves and their nieghbors.
Of paramount importance in this series of events, though, are the kingdoms of Cat, or Caithness, and Fibb. These two kingdoms would take control over near all of Pictland, likely by way of a combination of the Picts' matrilineal succession and war. The Picts followed lines of succession through their mothers rather than fathers. Legendarily, this is because the kings of Pictland were given wives by the Irish king Niall, but demanded the Picts follow lines of succession through their mothers, to ensure they always had a king of Gaelic descent. The interrelation of all Pict kings meant they were all eligible to inherit rule of one another's kingdoms, and ultimately Fibb came to rule all of the lowlands of Pictland, and Cat controlled most of the highlands.
It is unknown when exactly the two fell under control of a single king, though it may have been as early as Argentocoxos, but he may have only been king of Fibb. However, Drust mhic Yrp ascended the throne in 413 AD, and was king of both north and south. After his heir Talorg died, his younger brother Nectan Morbet mhic Yrb Mawr (the great) came to power. Nectan was possibly the first Christian ruler of Pictland. This fact may have laid the groundwork for the coming split between north and south. Nectan supposedly had to put down a number of pagan revolts in the north. Both his heir, Drust Guorthinmoc, and then Galan Arilith had to deal with similar religious revolts.
To curb such revolts, when Galan Arilith died, Drust mhic Udrost and Drust mhic Gyrom ruled jointly, one in the north, and one in the south, as one was a pagan and the other a Christian, though which was which is unknown. However, it is known Drust mhic Gyrom ruled a united Pictland for 5 years, but when he died, the kingdom was split between Fibb and Cat, though the official split would not occur until Drust mac Manaith died, presumably of the yellow plague, in 552 AD.
The history of the south is actually far spottier than the history of the isolated north. The succession of kings is largely lost. It is known that Brudei mhic Maelgwyn briefly ruled the south in addition to his rule as king of the north, through the puppet king Galam Cennelath. A little under a century after Galam's death in 580 AD, Northumbrians under Oswiu invaded Fibb, and killed the king possibly named Talorn. Fibb was conquered but for the land of Fife. Brudei mac Billi, king of Cat, recovered the south for the Picts.
The north was then ruled by the pagan king Brudei mhic Maelgwyn, son of the king of Gwynedd. He met Saint Columba, who freely converted and served numerous individuals in Cat's kingdom, including two of his sons. In 573 AD, he personally led his army to victory over a Dal Riadan invasion, and eventually commanded a brief vassalage of the Dal Riada. His heirs prepared to take control of the lands of Fibb, now under Anglo-Saxon rule, and in 667, began a war with Northumbria. The king Drust mac Donnel was defeated in battle against Ecgfrith, and was deposed by a rival claimant, Brudei mac Billi, who killed Ecgfrith in battle. Brudei defeated a massive Anglo-Saxon army at Nechtansmere, where he slaughtered the mass, and personally killed Ecgfrith, and proceeded to enslave or slaughter Northumbrians who inhabitted the region. His heir Taran was weak, and was deposed by Brude of Derelei, who became king of a once more united Pictland in 697 AD.
King Brude of the noble house Derelei was a powerful king, exerting his might to demand vassalage of both Dal Riada and Strathclyde briefly, though this was rescinded on his death. Nechtan mac Derile succeeded him, and brought the local church into union with Rome. In 737, a war with Dal Riada began under King Angus. Angus recieved aide from the kingdom of Ulaid, the northern Ui Neill, who had lost a few territories in Ireland to the Dal Riada. Angus defeated them in both Caledonia and Ireland, giving the territories in Ireland to Ulaid, and rendering Dal Riada in Caledonia as a province of Pictland, with his brother Talorgan being installed as chief.
Talorgan and Angus attempted then to conquer Strathclyde in 750 AD, and further expand the holdings of Pictland. A good initial campaign ended suddenly when Talorgan was killed in battle, most gruesomely as he was severed in two by a champion from Strathclyde, but not before he killed King Tewdur of Strathclyde. The Britons maintained the field though, while the Pictish army was broken and fled into the north. Angus, in revenge for the death of his brother launched a new campaign six years later, with aide from Northumbria. Again, the campaign started well, nearly capturing Dunbarton, but in an unrecorded battle outside of Dunbarton, the Northumbrians and Picts were slaughtered, and Angus had to flee back to Pictland with only a small entourage.
Dal Riada escaped the grasp of Pictish rule in a revolt in 768 AD, funded by Ulaid, who were fearful of a Pictish invasion. In 839 AD, King Eoganan of Pictland died in battle with vikings. Inheritance of rule, in an amount of irony, left the king of Dal Riada, Cinaed mac Alpin, ruler of much of Pictland, though the north remained under Pictish rule, until 'Mac Alpin's Treason' in 848 AD. In 841 AD, the Dal Riadan war with the Picts was at an apex, and Cinaed earned the title of 'Raven Feeder' for slaughtering countless Picts in excellently orchestrated battles. Cinaed invited his cousin, Drest mac Fethal, king of the northern Picts, and the remaining Pictish nobles to his palace at Scone, to discuss peace in 848 AD. Cinaed was one of the nobles who had a legal claim to the throne of the Picts. Every other heir was isolated with him at his capitol at Scone, where, in unknown circumstances, they were all murdered. The last Pictish king was dead, and Cinaed mac Alpin became the king of most of what is now the northern two-thirds of Scotland, though much was under control of vikings in reality. Pictish rebellions occured sporadically, but no Pictish kingdom would ever rule again, and their culture would eventually be worn out of existence under the weight of Gaelic and Norse influences in the north.