Soul Eater

The Soul Eater (ソウルイーター) is Riku's weapon in the Kingdom Hearts. 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. World of Chaos had a double bladed pole arm version of it in the final battle.

The Soul Eater was again Riku's weapon in Kingdom Hearts: Chain of Memories. The Soul Eater is also used by Vexen's Riku Replica and by Zexion since he has the ability to copy the weapons and attacks of others.

In Kingdom Hearts II, Riku uses the weapon to battle Sora in the Land of Dragons atop the mountain, and to fend off the Nobodies that had pursued Roxas to Ansem's mansion. He later refers to his Keyblade, the Way to the Dawn, as "Soul Eater", presumably as a force of habit.

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, Riku's Keyblade in Kingdom Hearts II, causing speculation that the Way to the Dawn was an "evolved" Soul Eater. However, in the secret ending video Birth by Sleep, which takes place many years prior to Kingdom Hearts, the Way to the Dawn is seen. It is unknown if this is the same version of the weapon that Riku acquires, but its existence suggests that the Soul Eater was designed based on the Keyblade, and not the other way around. Presumably, the Soul Eater could be an imperfect form of the Keyblade, since only darkness was used in this sword's creation. With Riku accepting both Light and Darkness, the Soul Eater could then assume its proper form.

Though later confirmed in one of Nomura's interviews that the scene with The Way to the Dawn in BBS was nothing more than a teaser and is not actually canon to the storyline. There for it's safe to assume that The Way to the Dawn really is an evolved soul eater. This theory is highly possible considering the fact that Soul is eater is a weapon made of riku's darkness, in theory is riku's darkness changed then soul eater would change with it. Though this is still speculation and can not be fully proven yet.

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.