Mystery boxes can be obtained from the Quiz Master random event. Players can choose between 1,000 coins or the mystery box upon successfully completing the event. Multiple Mystery Boxes can be owned at once, and they can be banked.
Puzzle box - OSRS Wiki
OSRS Wiki on X: OSRS fun fact #1: There are a handful of boxes, crates, and drawers scattered around the game that you can search through for a 1/1,000 chance to find
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