Major spoiler for the Elric saga:
Doing up the stats for Stormbringer would be up there with Statting out a god - because Stormbringer is the physical form of a Lord of Chaos. I'm not entirely sure how it merged with the Black Blade (the chosen weapon of the Eternal Champion) but at the end of the series Stormbringer walks away.
Other than that, Stormbringer (and its brother, Mournblade) is an awesome weapon. The sort of thing that can kill a god or be used to cut down an army. In many battles, the slightly nick from that blade caused death as the victim's soul was devoured - some of it being eaten by the sword and some of it used to heal and empower the wielder.
In short, it puts the Swords of the Cross to shame and is probably beyond the scope of the FATE rules set.
Richard
Here's why I think that the sword doesn't fit - and the spoiler is for the Elric novels. This specifically refers to book 4 - the Vanishing Tower (a book where Elric teams up with himself and himself and himself - which makes sense in the series).
In one short story, Stormbringer is used to kill a god. If I recalled correctly killing it wasn't the hard part - the hard part was dealing with the rush of life force the hero gained from killing the god.
We are talking about a "I swing at Mab - if I hit I kill her" type weapon here. A "I swing at Uriel - if I hit I kill him" thing. If you can hit it with a sword you can slay it and devour its soul.
I just can't see that fitting in the FATE system.
Richard