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.
