Soul Eater

The Soul Eater (ソウルイーター) is a form of Riku's Keyblade that appears in Kingdom Hearts, Kingdom Hearts: Chain of Memories, and Kingdom Hearts II. It is made of the darkness inside his heart, "forged" into its sword-like shape by Maleficent's magic. Riku wielded the weapon against Parasite Cage in Monstro, then against Sora in Hollow Bastion. Ansem wielded a double-bladed pole-arm version of the Soul Eater in the final battle between him and Sora. Even after obtaining the Way to the Dawn in Kingdom Hearts II, Riku continues to refer to it as his "Soul Eater".

A copy of the Soul Eater is also used by the Riku Replica and Zexion's clones, when he fights Riku.

Appearance
The Soul Eater is a scimitar-esque sword, with a red, purple and blue blade shaped like a demonic wing. Its handle bears a blue eye. It has a striking resemblance to the Way to the Dawn, as well as Vanitas's Keyblade. The specific resemblance to the Way to the Dawn implies that it may be a transformation of the Soul Eater.

Though the Soul Eater is frequently named a Keyblade, it lacks a Keychain, and has a much more sword-like appearance compared to other Keyblades.

Etymology
The weapon's name is derived from an ability attributed to the Dark Knight class in the Final Fantasy series, in which Soul Eater drains the HP of the user to initiate an extra-powerful attack. The attack and the Dark Knight class are associated with darkness, and the Soul Eater ability is sometimes called Darkside or Dark Wave, making it a fitting name for Riku's weapon.

Specifically, Cecil Harvey of Final Fantasy IV uses Souleater as his signature move, and both he and Riku fight with similar stances. Cecil's sword, like Riku's, has an eye on the hilt as well.