When you get to the abbey in Level 3, you'll be given a key to the rootbeer machine that opens the way to the catacombs area beneath the abbey. Whatever you do, don't leave the abbey until you've used the key. If you leave before you've used it, there's a possibility it could disappear from your inventory, never to return. If that happens, you can't get another one and you won't be able to open the door to the catacombs. You'll have to restart your game from the beginning or a previous save.
Taken with permission from Flying Omelette.