Used to unlock the Golden Chest in the city of Sanctuary, it will give a player access to the very rare loot contained within.
This very rare loot is scaled to a character's level. And because a player will only get one of these keys, it is crucial to consider what quality of loot they want to unlock.
While the loot itself can be traded or sold, the key cannot.
Preordering the game from a Premiere Club retailer or acquiring a code from an event or giveaway could earn you a key. Another way to get your hands on one is to purchase a Mechromancer DLC season pass.