It wasn't soul eater, it's blade was an angelic wing and the shaft of the key looked like a dragon's wing. I don't believe this is a prequel, unless the Heartless existed in that time period in which the knights are in.A few thins to notice too:
what was that thing disappearing behind the first knight?
Could all those keyblades represent worlds yet to be discovered, or already destroyed?
anyone ever think these 3 characters might be evil? that they might have killed the keywielders who one held the keyblades struck on the ground? they look so sauron-ish.
or pherhaps....could they be sora's, riku's and kairi's ancestors?!
and concerning the silhouette, is he a knight? an enemy or foe? or maybe it's Diz...kinda look like him.