The ruthless ruler of the Aztec Empire who was defeated by Hernán Cortés is Moctezuma II. Moctezuma was the ninth emperor of the Aztec Empire and reigned during the initial encounters with Spanish conquistadors in the early 16th century. His encounter with Cortés led to significant turmoil, as he tried to navigate the treacherous waters of diplomacy and warfare against the Spanish invaders, who ultimately overthrew him. Moctezuma II's downfall marked the beginning of the decline of the Aztec Empire and the rise of Spanish dominion in Mexico.