The Rusted Key is an item in Diablo 4 that you probably wouldn't think twice about, except that you drop it on the floor if you try to teleport out of the dungeon that it's found in. The key is an objective for Belfry Zakara, and like other quest items, it resides in the "key items" tab of your inventory until it's needed.
If you're racing to prepare for Diablo 4's first season, it's a good idea to hunt down all the Altars of Lilith so you can have those extra ability points to play around with on your new character. You should also make sure you've finished the campaign and explored all of the map too.
That said, if you venture into the Belfry Zakara dungeon and you're wondering what to do with the Diablo 4 Rusted Key, here's what you need to know.
Diablo 4 Rusted Key: What to do with it
The Rusted Key drops from an elite enemy found inside the Belfry Zakara dungeon. As mentioned above, it's one of the objectives of the dungeon and you'll need to loot this item in order to proceed through the locked door to the final boss.
As with all dungeons in Diablo 4, the layout changes each time you visit so I can't tell you exactly where you'll find the Anchorbound Chaplain, the enemy that holds the key, but once you've destroyed the two Drowned Bells for the first objective, it will appear on the minimap as a skull once you get close. Once it's been defeated, loot the Rusted Key from the ground, and you can then "use" it to unlock the door to the last boss.
Some of the confusion with this item likely comes from the fact that your character drops it on the floor if you attempt to teleport away from the dungeon with it in your possession—something you're likely to do if you need to empty your bags back in town. Once you take the portal back to the dungeon, you can pick it up again and carry on, but bizarrely, this seems to also happen once you've completed the dungeon and there's no further use for the Rusted Key.
Presumably, there is a bug that prevents it from being automatically removed from your bags once it's been used. So, if you hear that familiar loot drop sound when you teleport out of Belfry Zakara for the final time after beating the final boss, don't worry, you no longer need the Rusted Key and you should be safe to leave it on the dungeon floor.