It's not a halberd, it's a scythe.

(SPOILER)Anyway, CoM has two parts: Sora's story, and Reverse/Rebirth, which is played from Riku's point of view. As abrojtm said, Marluxia and Ansem are the final bosses of those games. Marluxia (the guy with the scythe) is the final foe faced by Sora, while Ansem is the final villain faced by Riku.

I'm sure you'll be able to find it somewhere. Or you could look for a game script or a plot summary if you still have trouble finding it.