Keyblade (Stygian Solace)

The Keyblade is a mysterious weapon against the Heartless.

Chapter 5: Meeting the Darkness
When Riku is swallowed by the darkness, a voice urges him to call "the key" to himself and to open the door. Riku goes to the door in the secret place, which is now adorned with a large keyhole, but cannot figure out how to summon "the key".