Aethelfirth (Aethelfrith in the Definitive Edition) is an infantry hero available in the Scenario Editor in Age of Empires II based on a Bernician King named Æthelfrith who ruled over present day Scotland. Æthelfrith was especially notable for his victories against the Britons and the Gaels. As a hero, Aethelfirth cannot be converted and regenerates health (since The Conquerors).
Aethelfirth is represented by a Woad Raider but with better stats. Like other infantry units, Aethelfirth is affected by all upgrades that affect infantry as well as any other bonuses that affect them.