The dusty note is an item found by searching the skeleton in the south-west part of the Tower of Magic prison during Tale of the Righteous. Although the item is optional, it provides instructions on how to solve the puzzle to open the gates that lead the player to the skeleton with the "investigate" option.
After investigating the skeleton, it will yield the message, "You notice some writing carved into the floor beside the skeleton: 'Lizards on the mountain. Do not trust the King. The altar is a lie.' " In a previous update, investigating the skeleton would instead give the player a dusty note with the message about the lizards being on the mountain and the king being untrustworthy instead of the current dusty note with the puzzle's instructions. The note can be kept after the quest, but it can no longer be reobtained as it serves no purpose beyond the quest.
Of all the traps in the Tower of Magic, this one has got to be the strangest. It's used to keep the cell gates locked, but it's like no lock I've ever seen. The crystals are the key. When all them are cleansed, the gates will open. The strange device can be used to cleanse them, provided it is powered with the right kind of energy.