Tristan is a cavalry hero found in the Scenario Editor in Age of Empires II HD: The African Kingdoms. He is based on a Cornish knight of the Round Table serving King Arthur with the same name and is also the hero of the Arthurian Tristan and Iseult story.
In-game, he is represented by an Magyar Huszar. Like other cavalry, Tristan is affected by all upgrades that affect cavalry as well as any other bonuses that affect them. As a hero, he cannot be converted and can regenerate health.
Despite his close association to British literature, it is unknown why developers chose to select his appearance based on a Magyar Huszar instead of a standard Knight.