A merchant selling hundreds of cardboard boxes for cheap, varying subtly in scale and dimensions but otherwise identical. They're not very useful for regular storage and are all named something long and borderline indistinguishable, like cardboardbox48.5cm*51.5cm*49cm. Mostly they're just good for putting down in great numbers for kicks, building fake walls around an NPC or pushing a tower of boxes off a cliff.

Elsewhere, in an entirely unrelated dungeon, lives an aggressive monster, large and deadly in a number of ways. Normally it's a regular (if challenging) side boss, but, if exposed to the correct box (randomised for each save file), it will placidly tuck itself into the comically undersized container and can be carried as an item.

This is where the fun begins.

Go to a well-populated settlement, whether a defenseless early-game home city or a sprawling late-game metropolis with its own military. Take out your Hell Box. Open it up and run to the nearest safe vantage point.

Watch wildly outmatched NPCs fight back as your absurdly out-of-place abomination starts a Godzilla rampage.

After finding and using the correct box, the merchant is mysteriously absent. If you go back to the dungeon where you found the monster, however, you can see her in the boss chamber. Attempts to interact will result in the merchant chuckling and walking into the darkness, where she despawns forever.