An adaptation of the Rod of Security from the 3.5e Dungeon Master's Guide.
Edward Perinse received an odd-looking rock as part of an inheritance from his father. It was about the size of a fist and colored a dull-looking red. He remembered watching his father poke and prod at it for hours trying to uncover any secrets it held. Edward had always thought his father mad for wasting his time on a rock. Until he learned two important facts upon receiving it when his father passed. The first was that it belonged to his ancestor, the notorious Black-Handed Knight and famed thief, Koryn Perinse. The second was that this rock was supposedly the key to his treasure trove.
No longer doubting his father's sanity, he began spending his free time examining the stone. Frustrated beyond belief one day, he smacked the stone hard against the wooden surface of his desk. Thoroughly caught off guard, he was teleported into a chamber filled with random junk. He had found out how to activate the damned thing.
The Stone of Security has a Will Saving Throw of DC16 for unwilling travelers, unless they're keyed to the stone. Keying people to the stone requires that the wielder be a spell caster, and for him or her to know the targets relatively well. In that case, the DC changes to 21.
The target(s) is teleported to a mini dungeon of 10 rooms. Once used a large storage space or hiding place, it is now used as an exam dungeon for the Ivory Tower, a place of learning for any who visit. Similar items have been known to exist, but none with so many rooms.
To leave, originally the person would have the stone and just have to smack it on a solid surface again. But when it was adapted to suit the Tower's needs, the stone teleports to the 'final' room for the exam takers to find and use to escape.