Wondrous item, rare
The key has 3 charges and regains all of them at dawn. As an action you can set the key to a lock that it plainly does not fit — a door, a chest, a press, a gate, a set of manacles — turn it, and expend 1 charge. The lock opens. If the lock is held shut by magic, it opens only if that magic was made by a spell of 3rd level or lower; otherwise the charge is spent and nothing turns.
The key leaves no mark. There is no scratch at the escutcheon, no filing, no burr, no broken ward, and a locksmith who examines the lock afterwards will swear it was opened with its own key and will be believed.
A lock opened by the key cannot be locked again, by any key or any spell, for 1 hour.
The key opens and does not close. While it is on your person, at each dawn one lock that you turned with your own hand the day before is found standing open, wherever it is and however far off. Nothing is taken from behind it. Nothing is ever taken.
It has been tried upon the iron door below the Long Room on four occasions and has not opened it. That door has two keys, and both are accounted for.
Source: Provenance — The Quist Sale