He was part of the generation of colonizers that began the first phase of Spain's colonization of the Americas.
Hernán organized a group of soldiers and scholars, and led them himself as they sailed for Mexico. Upon reaching the island of the Aztec capital, Tenochtitlan, he knew that his own Spanish troops were greatly outnumbered, and thus sought allies to help them negotiate.
Hernán eventually grew close to a Nahua slave named Malinalli, who spoke many of the major regional languages, such as the Aztecs' Nahuatl. She communicated for them, and though she understood Spanish, she could not speak it, leaving the Spaniard to only assume what she was saying.
Upon hearing of other foreigners on the island, living with the Chetumal Mayans, Hernán brought his troops to search for them. Eventually, they met up with fellow Spaniards Gonzalo Guerrero and Gerónimo de Aguilar, who had been shipwrecked on the island a decade previously.
With their help, Hernán was able to communicate with the Tlaxcalans, an indigenous people not yet conquered by the Aztecs, so as to help gain entrance into Tenochtitlan itself.
When Hernán and his troops finally entered the capital, by Guerrero's suggestion, each man took off his helm, and those with the fullest beards stood in front - a show for the locals, to have them believe that the Spaniards were gods.
The ruse worked, and Hernán's soldiers were granted an audience with the Tenochtitlan leader, Moctezuma II. He greeted them as "our lords come on earth", and invited him to stay in his own chambers.
Come the Festival of Toxcatl, the Spaniards attended the celebrations. However, upon witnessing a brutal human sacrificial ceremony, Hernán's men retaliated; some out of horror at what they had seen, others as an excuse to steal Aztec gold.
The Spaniards slaughtered several unarmed nobles, many of whom only remained confused as to why their "gods" were attacking them. However, upon seeing some men looting the bodies, the illusion broke, and a battle ensued.
This massacre, which eventually drove Hernán from Tenochtitlan, would later be known as La Noche Triste, or the Night of Sorrows.