The key was passed along generations along the line of the chosen one, until one of the ancestors, Hégésippe, was attacked and killed by the pirate LeBorgne, stealing his key and adding it to his treasure.
After reading a message in the cave of his house of how the key got lost, Twinsen finds out in a book in the library at Principal Island, that the location of the treasure is in the Maritime Museum of Proxima Island. Twinsen then has to find out a way of robbing the key from the museum.
"Ancestral Key. Opens the secret compartment in Twinsen's house."