The King is a special untrainable unit introduced in update 14825 of Age of Empires II: Definitive Edition - Return of Rome, appearing in Regicide games. They cannot do any task, but can be used to scout in the early game when the threat of being attacked is low. In Regicide games, players that lose their King instantly lose the game.
The King reuses the voice lines of the male Villager.
The King has the same model, no matter the civilization's architecture set. The model's beard, outfit, and sceptre seems to be based on Mesopotamian, such as Babylonian and Assyrian, kings, but features a metallic laurel wreath as was worn by Roman emperors.