Just the stress track, I'm afraid.
Though whether the loss of powers is mandatory or just another option alongside consequences to 'soak' the stress is a matter of debate.

Yea I just noticed a post earlier in the topic actually mentioning it. My bad.

In any case your going to start loosing your powers once your out of stress and consequences your going to loose powers. (unless you want to be taken out...) So even a player who cant help but use his powers is going to start loosing them and make his defence roll easier. Still will need to recover those powers tough.

Ok, to be honest I am not completely satisfied yet with what the agenda for necromancy would be. So I was wondering if anybody could write up what they inmagine as an interesting compel from this sponsor?

I am having a bit of trouble wrapping my head around some of the sponsored magic flavours. Soul-fire requiring goodynies I can live with, but Kemmlerian Necromancy really ticks me of. What source of power does it draw on, specifically? I can only think of death itself and I just don't see death as having so much of an agenda.
I feel that if an bad guy is using sponsored magic this should limit him somehow, allowing the players to get an edge in once they recognize the pattern, but what does kemmlerian force a necromancer to do what he would not have done already?
The same goes for the layline sponsor in the neutral grounds case file. I understand that some sources might be more mysterious and knowledgeable, but that seems to make them also unplayable or at least flavourless and bland when they come in to play.

Anyhow, I am sure this has been addressed in the past, I just haven't been able to find it yet.

The way I look at it, a Valkyrie-Scion's powers would be more likely to function only against "supernatural" damage, rather than against anything but "supernatural" damage.

I was going to say this as well, If you want your catch to be of this nature, only working against X, I would agree that a far more interesting catch would be that they work only against the supernatural.
If anything this makes for a far more survivable character AND a more interesting game, for the GM throwing mortals at you will be just as big a threat as monsters so he can mix it up a lot more, and you get to use your powers when the matter the most.
Your powers have no grip on mortal men as you do not have the right to chose the fallen, only being a descendent, but against the supernatural that rule doesn't work.

Thanks for the replies guys. (and the warning, sinker, 28 pages, damn)

I think the main concern the player and me by extend had was that it would seem to be very powerful effect in respect to some other aspects the book has. Something one of my players came up with was that it could be like a -1 power to create fallout, a bit like mana static that allows hexing.

For a more story based view. A power is something you can do. Its a ability you have, personally. Sometimes you are only conducting the power (sponsored magic) or it was given to you unwillingly, but using it in game is always the characters choice to employ it. The character decides to cast the spell or use his claws or whatever. Sometimes its the player accepting a compel but in game its still the characters choice rather then being forced (tough he might be acting according to nature rather then free will) This curse however is not a ability in that manner. There is no choice from the character to make it happen. The player could offer a fate point and ask if something wacky could happen but the character could do nothing more then pray for it to do so, he himself has no control whatsoever over it. I think that is an important distinction here, tough I can imagine somebody coming up with a list of powers that cross that line as well, I cant think of any.

As mentioned it also allows me to screw the players over royally and I could encourage the entire table to participate in coming up with the weirdest thing that could happen, so in the end if it makes the game more fun for everybody what's the problem? 

I do think that with such a explosive effect I would want it to be part of his high concept tough. But that's a minor detail.

Well you can compell it really easy which will give him the fatepoints to have a lot of luck. Also easy to selfcompell.

This is true, but I am wondering if its not creating to much effect for an aspect. I mean I could take "I can grow back an arm" as an aspect but unless I actually take a power to do so I probably should not be allowed to actually grow back an arm.
By the same logic I fear this would be to big a power to allow only from a aspect. Basically using the aspect for such an event would be a declaration, and the aforementioned example seems a bit to powerful for declarations.

Yea I know, but to elaborate a bit, should a fire evocation fail small things like buildings might burn down. Yet the spell still might work, with veils, which are supposed to hide the caster or an object any fallout might rather attract attention to the object, entirely preventing the point of the spell in the first place. I suppose stuff like "see and be seen, in reverse, invisible and blind" could work for fun, but its still somewhat limited.