Eternal Spire of Oolmagi

A mirror brought back by adventurers, touching it transports you into a challenging tower filled with monsters that contains about 25 floors.

10th floor is still max anyone has achieved. If you die inside before time runs out, you return outside alive. Only 5 or so people who wanted to go in at once, so no known maximum. Each room starts with 5 min., and each successive room starts with an additional 5 min.