Rusted Key

The Rusted Key is an unlockable common item in Risk of Rain 2. When a player has this item, a Rusted Longbox spawns somewhere in the next level. The Box contains an Item, the rarity of which is determined by the amount of rusted keys the player has when they open it.

The Rusted Longbox is quite small and easy to miss, but it's dark color and rectangular shape usually make it stand out. It can be opened by players who don't have a key, but they will almost always get a white item. Opening the box costs nothing, and will trigger Bundle of Fireworks.