"Graceful Assassin" is an instrumental piece composed by Yoko Shimomura. It was first featured in Kingdom Hearts Chain of Memories. It is also Marluxia's title in Organization XIII.
In Kingdom Hearts Chain of Memories, Kingdom Hearts Re:Chain of Memories, and their respective HD remakes, this piece plays while Sora fights Marluxia's illusion in the first of two battles against him.
Disc 8, Track 19 in the Kingdom Hearts Original Soundtrack Complete