This keyblade is used by Roxas along with the Oathkeeper. It can be found by Sora when you open the chest located in the room where Riku was fought while possesed by Ansem.

These keyblades are both used by Roxas. Sora can weild them both too, at the same time. Equip them using either Valour Form, Master Form, or Final Form. (In Valour Form, his fighting style is simular to Roxas's)

modified oblivion keyblade art