Brittany, last bastion of the Celts on the Continent, juts like a bony finger into the Atlantic, all wind-swept rocks and misty forests. Its people are hardy fisherfolk and warriors, bred of the Celtic raiders of old and the refined Frenchmen to the east. In times past, Brittany was nearly impregnable. The land and the people were of the same sort, independent and even a bit wild.
The Bretons chose their own rulers. An old Breton saying goes, "Only a Breton may rule another Breton. Only a family of Bretons may rule all of Brittany."
Of those families which governed the peninsula in centuries past, one stands at the forefront of history: the de Rennes family. Ensconced in their ancestral castle at Rennes, they controlled a patchwork of small counties and underdeveloped land with ruthless cunning for six hundred years.
The progenitor of their line was a man named Judicael. He signed on with Viking raiders in the mid 9h century, establishing a reputation for savagery even by the standards of the Northmen. He ruled nothing beyond what he could see from his castle, but his coffers were fat with the spoils of a lifetime of raiding. The Northmen called him "Bror", seeing him as a kindred spirit despite the differences of lineage.
His grandson, Conan, married Ermengard, daughter of the Duke of Anjou. With the powerful d'Anjou family allied with the de Rennes, Conan established himself as Duke of Brittany at the head of an army of mercenaries and Frenchmen. Most Bretons rallied to his banner immediately, seeing him as the man to unite the peninsula. Those few who stood against him were swept aside and replaced by his family members.
The next two generations were spent consolidating the rule of Brittany. The alliance with Anjou faded with time, leaving Brittany bereft of allies but strong. Conan I's grandson, Conan II, would prove to be a remarkable ruler, one who would change the history of Europe far from the bounds of his homeland.
(I'm doing an AAR, or after-action report, for Crusader Kings II and this is a good place for it.)