Underworld Key

The Underworld Key is an object that debuted in Kid Icarus: Uprising. First appearing in Chapter 7, it is a purple gem with the insignia of the Underworld Army in its center, used to gain access to the Underworld.

Chapter 7: The Seafloor Palace
In order to reach Medusa, Pit must first obtain the Underworld Key to pass through to the Underworld. Palutena sends Pit into the Seafloor Palace, where they attempt to negotiate with the holder of the key, Thanatos. However, the God of Death only dodges the subject, forcing Pit to seek him out in person instead.

Once Pit arrives at the boss room, he engages Thanatos in a long battle. Upon his defeat, Thanatos bursts into light, leaving the Underworld Key behind in his place. Pit then picks it up and briefly observes it before Palutena whisks him away from the palace.

Chapter 9: Medusa's Final Battle
Having obtained the Underworld Key, Palutena guides Pit through a ravine where the entrance to the Underworld is located. Once he gets close, the gem begins to glow, and Pit successfully warps into the Underworld.

Underworld Key
''Held by Thanatos, this jewel unlocks the entrance to the Underworld. Although the Underworld and Overworld seem close, they are quite difficult to cross between. If free passage were allowed, there would be no telling the living from the dead.''