That's One Winged Angel, the keyblade you get for beating Sephiroth.
It's got 8 atk -3 mg but +12 Criticals (it's only rivaled by La Vi En Rose/Divine Rose).
And yeah Xemnas will own the fuck out of Donald and Goofy, truth be told I was trying to get them killed so they wouldn't screw up my loop. I do believe your theory is right as well, it's just a way to test Sora as far as Roxas' capabilities are concerned.