Catacombs Key

The Catacombs Key is used to open Floor 12 and do a dungeon run. The Key is obtained through Multiple Floor 11 runs and is guaranteed to be in your final loot table once you finish your first Floor 11 run. It takes a while to be able to beat it, but once someone beats it and starts using the weapons from there, it is going to get easier from there.

You can also craft an Enchanted Catacombs Key which has infinite uses. It takes a lot to craft it though. The Enchanted Catacombs Key counts as an Artifact so you would need to put it inside your Artifact Bank.