Sandalwood Box

The Sandalwood Box is a magically sealed box that can only be opened by a few. It is in the possession of Lord British, who uses it to store his most prized possessions. It appears in Ultima V and Ultima IX,.

Description
Before commencing his journey, Lord British stored his Orb of the Moons in the box, hiding it behind a secret wall in his bedroom. Blackthorn actually knew of the existence of the box and its contents, but not how to actually find it so that Lord British could never return. Thus he sent the spy Saduj into the castle to find out where it was hidden and then obtain it. The Avatar in Ultima V found and took possession of the box by playing "Stones" on the harpsichord in his Lord's chambers. Later, in Dungeon Doom, Lord British opened the box to use the Orb of the Moons in it to escape from the dungeon through a red moongate.

In Ultima IX, the Sandalwood Box contained the Ankh of Spirituality, and again had to be opened by playing the song, "Stones". The Avatar could find a hint about that in one of the books in the castle library and then ask the court musician Siona about the notation, so the harpsichord could be played.

Ultima V: Lazarus
It is easy to lose the box forever in Ultima V: Lazarus. Saduj steals the box from the Avatar if being in the party before fleeing. If the party is caught in the Palace of Blackthorn, Lord Blackthorn takes it away from the Avatar and murders a companion. Both instances mean a game over.

Trivia

 * An attempt to use the Sandalwood Box in Ultima V results in the Avatar asking, "How?"
 * Not having the box or not admitting having it to Lord British (regardless how insane that is) results in a non-standard game over, resulting in the heroes being trapped with Lord British, most likely waiting to starve to death. It's made worse by how little clues there are about the box.
 * Actually getting the information from Saduj after giving him the Oppression password is the most direct hint one can get.