Eh, to me, it did work; you forced the bad guy to spend some of his fixed resources to overcome an obstacle. Depending on the badguy, that's more serious a loss than some consequences. Thats victory... especially if you have a parties worth of Fate points, and he has the one or two he got Conceding out of an earlier conflict, or by accepting a painful compel.
If the aim is to keep the badguy from filleting and barbecuing you with Weapon:8 spells round after round, and he spends one fate point to keep filleting and barbecuing you with Weapon:8 spells rund after round, then it didn't work.
The players are casting this spell with an objective in mind that
isn't "make him use one fate point," so to me, if that's the only result then it's a failure. The villain isn't dangerous to them because he has that one fate point, he's dangerous to them because he's a powerful magic user. And being able to cast your full load of spells is pretty much always worth a fate point if the alternative is not being able to cast.
If the scenario is about putting together a 'bad guy defeating spell', then he shouldn't buy out of it if thats what makes the story work Of course, thats kindof harsh if it results in him being helpless as the party stomps him to death afterwards, as he lies there.
Honestly, if it were me, and the plot allowed for this sort of spell to make things work... I think the villain should Concede immediately, somehow. He's already lost if his primary power is sealed, and there's really no reason to run a conflict at that point. So I guess that makes this sort of thing more of a 'Challenge' per the book, and less of a Conflict to begin with, right?
Well, presumably a good villain has
some other way of defending himself beyond magic (Kravos had his demon and cultists, for instance)--magic just happens to be one of the options that offers the best offense.
Depends on the scenario, of course. I was thinking of it in terms of, "The players are casting this spell to give themselves a fighting chance," like what Ebenezer did to Mavra, and what Harry did to Kravos.
If you're casting the spell as an 'instant win' button, then hell, just make it a straight up attack and take his ass out.