- "I can supply you with a regiment of Grail Knights for as long as you are within Bretonnia. Isn't that generous enough?"
- —King Louen Leoncoeur pledges his Grail Knights.
Tristan was tasked by King Louen himself with assisting a mighty alliance comprised of troops from the Empire and Kislev, alongside Dwarfs and Wood Elves. Led by the Mercenary force known as the Grudgebringers, this massive army would be sent to the dark city of Mousillon, tasked with destroying one of the Old World's greatest threats, an army of the Undead, led by the resurrected corpses of Tristan's fellow Grail Knights.
Tristan and his companions proved to be the most powerful troops in the coalition army. Having sipped from the Grail itself, they were the most powerful of all the Knights of Bretonnia. Their skill in combat was seen as extraordinary, and they fought determinedly. When fighting against the Undead hordes of the Dread King, Tristan and his Grail Knights were the only warriors who showed no fear. Indeed, they were all but immune to such types of psychology.
In what would become known as the Battle of Mousillon, Tristan and his allies fought against the undead hordes that spilled from the city's dark gates, and eventually engaged the Black Grail itself. The coalition army was ultimately successful in defeating the Black Grail Knights, although at the cost of many lives. With his task complete, Tristan led his men back to Parravon, with the eternal gratitude of both the Grudgebringers and Bretonnia itself.