Thierulf of Lyonesse was one of the first Bretonnian Grail Knights and the first and most devoted of the Grail Companions. After the founding of Bretonnia, he became Duke of Lyonesse. More than a century and a half later, he died in the Battle of Amandur.
He and his brother are said to have slain a lion in childhood. With the lion's fur over his shoulders and head, he rode out of Lyonesse as adult to join with Gilles of Bastonne who was riding out to beat back the evils that beset the lands east of the Grey Mountain. Thierulf had wedded Gilles' sister Rosalind years before and was a good friend of Gilles. Together with Landuin of Mousillon, he united with Gilles at the edge of the Forest of Châlon and they were visited and blessed by the Lady of the Lake who appeared above the water of a lake with the Grail in her hands. Thus they became the first Grail Knights. The three set upon a conquest and, winning twelve battles in a row, they founded Bretonnia. Each Bretonni tribe that was freed, sent their Lord to join in the conquest and receive the Blessing of the Lady. Thus was formed the Grail Companion. Thierulf is said to have fought with the ferocity and heart of a lion. Many tales are spun about the Grail Companion. Of Thierulf is said that at the Ninth Battle he restled with a two headed giant.
After the founding of Bretonnia, Thierulf was appointed Duke of Lyonesse. When Gilles died sixteen years later, fighting greenskin in the Grey Mountains, Thierulf was mentioned as his successor, but eventually Louis the Rash, son of Gilles, was given the preference. When Louis departed on his Quest for the Grail in order to proof his ability to occupy the throne, Thierulf acted as steward untill Louis returned a Grail Knight years later.
Landuin and Thierulf fell into an argument over Rosalind, Thierulf's wife, and a challenge was fought. Landuin came out victorious, inflicting a wound across Thierulf's face.
Only 163 years after the founding of Bretonnia, Thierulf, whose lifespan had been extended by the Lady, fell in battle. It was at the side of King Guillaume at the Battle of Amandur against a horde of Orcs.