Night of Sorrows

The journey to Mexico was an event in 1520, during which the Assassin Giovanni Borgia traveled to Mexico on a Spanish ship under the command of Hernán Cortés, in an attempt to find one of the Pieces of Eden.

Giovanni Borgia pretended to be a Spanish scholar to be allowed entrance onto the ship and get a chance to retrieve a Piece of Eden, which turned out to be one of the Crystal Skulls.

However, the journey resulted in a massacre of the Aztec people when the Spanish explorers grew greedy and were offended by the Aztec rituals.

Journey and arrival
Knowing that there had to be a Piece of Eden somewhere in Mexico, the Assassins used their contacts to place Giovanni Borgia on a Spanish ship, making the crew belief he was merely a scholar. During the journey, Giovanni wrote a journal to inform his fellow Assassins on his journey when he returned, though he made sure no one else would notice anything suspicious about the journal.

Upon their arrival in Mexico, they started contacting the local tribe with the use of several translators, making communication between the two parties slow and troublesome. They were eventually taken to Tenochtitlan, where Giovanni was astonished by the size of the city and the amount of citizens. They were soon greeted by the leader of the tribe, Moctezuma, who believed them to be Gods and allowed them to stay in his palace.

Massacre
Believing the Spaniards to be Gods, the Aztecs started their celebration to honor them, having citizens dress up as Gods with expensive jewelry and gold. The Spaniards, however, grew greedy when they noticed how much gold the Aztecs had. Meanwhile, Giovanni Borgia started his search for the Piece of Eden, though he had no idea what he was looking for. The Spaniards eventually ended up in a temple where the Aztecs were about to sacrifice on of their own men, which left the Spaniards disgusted.

After the sacrifice, which involved killing a man in a seemingly brutal way, the Spaniards drew their weapons and started killing the unarmed Aztecs that were honoring them. Giovanni then noticed a priest upholding a crystal skull, and managed to assassinate the priest and claim the skull without anyone noticing.

The Spaniards were driven back to Moctezuma's palace, where they convinced the leader to speak to his people and calm them down. Moctezuma tried to convince his subjects to stand down and return to their homes, but they refused and started throwing rocks at their leader, who was killed by one of them.

Escape
Having been locked up in the city, the Spaniards started preparing their escape. They built a portable bridge to cross the rivers and took as many gold as they could, not realizing the weight of it would slow them down.

They sneaked through the night and were convinced they would escape without any trouble when one of the captains was hit in his eye by an arrow. However, the Spaniards managed to fend of the Aztecs and many of them escaped back to Europe.

Aftermath
Having recovered the Crystal Skull, Giovanni travelled to Basel and allows Bombastus to study it. Bombastus, however, could not make the skull work as he had hoped and the mystery of the skull would haunt Giovanni for the rest of his life.

Source

 * Assassin's Creed: Project Legacy - Chapter 2 - Giovanni Borgia