Oblivion

The Oblivion is a Keyblade in Kingdom Hearts, gained by opening a chest located in the room where Riku was fought while possessed by Xehanort's Heartless. The player doesn't get a chance to open it until after the fight. One of the strongest melee-oriented Keyblades in the game, Oblivion reappears in Kingdom Hearts II, as both a keychain for Sora after his reunion with Riku, and as one half of Roxas' dual set of Keyblades, the other being Oathkeeper. It can be seen as a Keyblade of darkness, to balance out the light of Oathkeeper. The teeth are fashioned in the shape of the Kanji for "darkness" (闇), corresponding to the Oathkeeper's Kanji for "light". However, the location where Sora first obtains it in the first game, and its name suggest a representation of not just Sora's friendship with Riku, but also Sora's sacrifice for Kairi. The keychain is found in the first game in the same room where Sora sacrifices his heart to save Kairi's, thus casting himself into the darkness. Also, the name is reminiscent of Castle Oblivion, and the design of the shaft looks like an Organization XIII robe. The design could also represent the creation of Roxas, who became a member of Organization XIII.

In Kingdom Hearts: Chain of Memories, the Oblivion card is obtained on the 12th floor after completing Destiny Islands, and after defeating Larxene for the second time.

Appearance
Oblivion is similar in shape to the Kingdom Key, though is deep black and has a longer reach. The hilt guard is comprised of two bat-like wings extending downward. The teeth are in the shape of the Kanji for "darkness" (闇). The keychain motif is a black version of Sora's crown necklace.

Trivia
If looked at closely at one point in a secret movie unlocked at the end of Kingdom Hearts, Oblivion seems to have a chain in the center of it. This is likely a reference to what the dark did to Riku (as this keyblade is Sora's representation him): It chained him with the power of Darkness.