Keyblade of Heart
The Dark Keyblade was made by Xehanort's Heartless with six of the seven Princesses of Heart's Hearts.
In Hollow Bastion, Riku under the control of Xehanort's Heartless used it to unlock Maleficent's heart. This gave the witch the strength of her inner Darkness, turning her into a dragon. Sora defeated Maleficent her again while she was in this form and then proceeded to find the possessed Riku.
Near the Final Keyhole, Xehanort's Heartless, through Riku, told Sora about Kairi's heart residing within him and then proceeded to attack and release Kairi's heart from Sora. However, Sora defeated his possessed friend and then turned the Dark Keyblade onto himself to unlock Kairi's heart. The stab from the Dark Keyblade turned Sora into a Shadow. This created his Nobody, Roxas, as well as Naminé, who was born from both Sora and Kairi.
The Dark Keyblade then vanished, turning back into the other six princesses' hearts. To this date, it hasn't been seen in any other games in the series.
