Fountain of Youth

The Fountain of Youth was a fabled fountain, said to give eternal youth to anyone who drank its water.

The Assassins knew that this was because of a Piece od Eden located in the fountain.

History
Several explorers tried to reach the fountain. One of these was Juan Ponce de León. The Assassins weren't successful in turning him back so they killed him.

Source
Assassin's Creed: Discover Your Legacy