The skull tune was a simple type of musical combination of notes to be played on an instrument made of skulls. It would trigger an opening mechanism, unlocking or opening a door to the Tomb of Sir Richard in the Venetian catacombs.
Abbess Hildegard of Bingen saw the tune in one of her visions and transcribed it with the note Per Hos Sonos Sepulcrum Aperies ("By these sounds you will open the tomb"). Her manuscript was kept in the library of the Abbey of Sankt-Gallen with a description of the Holy Grail.
Behind the scenes
The combination is random in each new game.