Thierulf of Lyonesse was the first of the mighty Grail Companions, holy knights of immense power who united Bretonnia alongside Gilles le Breton. His loyalty to Gilles was unmatched and he fought always at his side.
Thierulf, lord of Lyonesse, took a lion's head as his heraldry after he and his brother slew a lion as children. From that day forward, each time he rode to war, he wore the lion's fur up over his shoulders and head, and it is recorded that he fought with the ferocity and heart of a lion.
Thierulf was the lord of the Bretonni of Castle Lyonesse and the surrounding lands by 950 IC, when he married Rosalind of Bastonne, sister of Gilles le Breton, and he became a strong friend of the heir to Bastonne, while the ties between the two realms grew strong.
Twenty-five years later, Gilles' father died and he joined forces with Thierulf and Landuin of Mousillon to repel the greenskin armies attacking their territories. Soon thereafter, Gilles and his friends were visited by the Lady of the Lake, who blessed them and turned them into the first Grail Knights.
The Companions spent the following two years campaigning throughout the lands of Bretonnia, joining forces with the other eleven lords of the realm and fighting a series of clashes with greenskins, Undead, Skaven and Beastmen that became known as the Twelve Great Battles. They were victorious in each, and earn much honour, glory and renown. The deeds of the Companions became the epitome of aspiration for all knights, and these battles formed the basis of countless Bretonnian tales in later years. Thierulf was particularly renowned for wrestling with a monstrous two-headed Giant as he defended one of the walls of Castle Mousillon in the Ninth Battle.
After the securing of all the lands of the Bretonni tribes, a great meeting took place in the home of Folgar of Artois. Here, the formal dukedoms were created, and the Bretonnian calendar was introduced - thus, year 979 of the Imperial Calendar is considered year 1 of the Bretonnian one. Each of the great lords of the Bretonni, the fourteen Grail Companions, including Gilles le Breton, were named Dukes. The Dukes swore oaths of allegiance, and Bretonnia was formed.
In 1003 IC, Landuin and Thierulf, always competitive, had a major falling out, with some believing that the cause was over Thierulf's wife, Rosalind. A challenge was fought between he two mighty warriors, with Landuin coming out victorious, inflicting a wound across Thierulf's face.