The Underworld Key, guarded by Thanatos, is an important relic in Kid Icarus: Uprising. The key opens up a now closed pathway to the Underworld. The god of death safe-keeps it inside his belly, an important task only the second-in-command is given. Unlike past games where entering the Underworld was possible by anyone who survived, a key is now needed.
The key first appears after the boss battle of Chapter 7, The Seafloor Palace. Once Pit defeats the god of death, he will explode and release the Underworld Key he kept inside him. The angel takes it and is then returned to Palutena. Once the Three Sacred Treasures are re-equipped, Pit descends into the portal using the key in Chapter 9, Medusa's Final Battle. In the key's more detailed Idol appearances, it is shown that the gem has an Underworld Army logo imprinted inside it.
Held by Thanatos, this jewel unlocks the entrance to the Underworld. Although the Underworld and the 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.