The Bell of Opening is one of the Invocation Artifacts in NetHack. You will receive it after defeating your Quest Nemesis. As with the other Invocation Artifacts, it cannot be destroyed or placed inside a container.

If the Bell of Opening has no charges, applying it is the same as applying any bell: if noncursed, ringing the bell awakens nearby monsters and calls your pet(s) back to you. If the Bell is cursed, there is a 25% chance that it will summon a nymph (if they're not already extinct or genocided); this nymph will not be carrying anything. There is a small chance that you will also be paralyzed or the nymph will be fast as if zapped by a wand of speed monster. There is no chance that ringing the Bell of Opening will cause it to break (this is a problem with ordinary bells).

If the Bell of Opening is blessed, and has at least one charge remaining, it will cause nearby doors, chests, boxes, etc to be unlocked and opened. It also finds nearby secret passages and some types of traps. If you ring the Bell while you are inside a monster, you will be expelled immediately. This action saved my life once when I'd been swallowed by a purple worm.

If the Bell of Opening is uncursed, and has charges remaining, it expels you from being inside a monster if applicable; otherwise, it has the same effect as a charge from a wand of secret door detection.

If the Bell of Opening is cursed, you won't be expelled from inside of a monster. If you aren't swallowed, it awakens nearby monsters and summons some undead creatures for you to play with.