Infernal Puzzle Box

From Baldur's Gate: Descent into Avernus, page 224.

Description
Wondrous item, uncommon.

An infernal puzzle box is a cube-shaped container 5 to 6 inches on a side, composed of airtight, interlocking parts made from materials found in the Nine Hells. Most of these boxes are made of infernal iron, though some are carved from bone or horn. Infernal puzzle boxes are used to safeguard diabolical contracts signed between devils and mortals, even after the terms of these contracts are fulfilled. An empty infernal puzzle box weighs 3 pounds regardless of the materials used to fashion it.

When an object small enough to fit inside an infernal puzzle box is placed in it, the container magically seals shut around the object, and no magic can force the box open. The sealed box becomes immune to all damage as well. Every infernal puzzle box is constructed with a unique means of opening it. The trick to solving the puzzle is always mundane, never magical. Once a creature figures out the trick or sequence of steps needed to open a particular infernal puzzle box, that creature can open the box as an action, allowing access to the box's contents.

A creature that spends 1 hour holding an infernal puzzle box while trying to open it can make a DC 30 Intelligence (Investigation) check. If the check succeeds, the creature figures out the trick or sequence of steps needed to open the box. If the check fails by 5 or more, the creature must make a DC 18 Wisdom saving throw, taking 42 (12d6) psychic damage on a failed save, or half as much damage on a successful one.