Final answer:
Moctezuma II was the Aztec emperor when the Spanish conquistadors, led by Hernán Cortés, arrived in 1519.
Step-by-step explanation:
The Aztec emperor in power when the Spanish conquistadors arrived was Moctezuma II. Hernán Cortés, a Spanish conquistador, led the expedition that would eventually lead to the fall of the Aztec Empire. Upon his arrival in 1519, Moctezuma II greeted Cortés and his men with gifts, as he mistook them for divine figures. However, Cortés later took Moctezuma II prisoner and used him to gain control over the Aztec people. This tumultuous period ultimately ended in the destruction of the Aztec capital, Tenochtitlán, and the establishment of Spanish control over what is now central Mexico.