Consider a character with the Strong Faerie Blood Virtue, where the faerie in question is a dwarf (dvergir) how would you set the mechanics of a Greater Malediction having to with turning to stone in sunlight (as the dwarves of norse lore were wont to do). I'm thinking something that is not permanently lethal, but can very well entail not geting anything done while out and abouts.
Sadly I seem to be coming up blank in regards to the mechanics, so any input'd be appreciated.
By "the mechanics" what do you mean?
Turning to stone magically does not, as I understand the hermetic system, actually kill the target. It merely renders their conscious form inert until the effect wears off.
Any dwarf with such a malediction (especially if known by the dwarf) would simply force him to go out at night if at all necessary. If he's stupid enough to blunder out in daylight then I would simply let him adorn the side of the road as a curiosity (and a perch for pigeons) until sundown. Just hope that such a stupid dwarf would not do so near to any stonecutter's lodge lest he be sold as a decorative piece or worse (chiseled up into scrap) lol.
The latter consequence WOULD be his demise.
Other than that, from what I read of Strong Fairy Blood, there is nothing that would preclude such a malediction nor shape its effects in any way other than those specifically laid out in the malediction itself.
Nono... Of course not. My question might have been vague.
The character has Strong Faerie Blood (Dwarf) AND Greater Malediction. The problem is that Greater Malediction is an incredibly vague Flaw. The only example I have seen is in HoH: TL under Bloodcaps.
The question quite simply is this:
What would be fitting game mechanics (the hard rules) for an effect that causes the character to become stone in sunlight. Not instantly turning to stone as long as the sun is in the sky.
I was thinking more that as long as the sun is covered by clouds or he's indoors, nothing happens. (except being sort of paranoid about being caught around stone carvers when the sun comes out after rain)
Ah, well in that case your mention of Strong Fairy Blood was rather superfluous, but no matter.
As far as you've clarified, I would say make the condition of the malediction "touched by direct sunlight". This will provide the margins you seek for cloudy skies, dense forest, dwellings, etc. (any sort of cover from direct sunlight).
It still leaves room for the SG to have some other capricious fairies trick him into the sun (by making a temporary cloud cover and removing it at just the wrong moment) or even dragging him into it, as a form of malicious practical joke.
Other than that there are no real "mechanics" (i.e. rolls or such) to be considered, that I am aware of. Im sure one of the other many fine people here will correct me if I'm wrong.
Hmm, faerie magic, so no logic involved I would say. I like your idea, consider adding Nocturnal to your character.
I think if nothing bad happens to him while in stone shape it should be a minor flaw.
Yes I agree, I was thinking of that after I my previous post.
Greater Malediction tends to imply something grievous and longlasting happening to the person suffering from the curse (or those around him even, i.e. Cows drop dead when he passes or crops wither in the fields, etc.).
A purely personal inconvenience is definitely a minor issue.
Yes, I agree with the emerging consensus: it should be a minor flaw, as it doesn't hamper him that much (especially as a magus can fairly easily block out the sun's light on himself).
For more specific mechanics, I'd say...
You turn to rock when hit by direct sunlight, and for as long as you are. Even a simple shade prevents this, as can magic that darkens the area you're in or so on.
This is a Minor Flaw.
Follow "The Silent Vigil" (page 132): You can only hear, rolls to recover Fatigue or wounds take twice as long to make.
Unlike the Hermetic spell, I'd say your cloths and personal gear is also changed with you. Seems more appropriate to the image of a dwarf freezing as the rays of sunlight hit him, no?
Note that you can still cast spells, but without gestures and words. It isn't possible to change your shape while under the curse, however - not for you or any other magic.
If hacked to pieces, you die. If parts are cut off, you lose them (becoming Lame, Missing Hand, and so on), and also suffer a (usually Incapacitating) wound.
I'd recommend taking a flaw such as Incompatible Arts (Perdo and Rego Ignem), or Deleterious Circumstances, or even Restriction.
If your magic is totally unable to affect sunlight and you're unable to cast magic while in stone (perhaps unable to feel or think, too), I'd count this as a Greater Malediction.
I'd rule slightly differently. Its my understanding that if turned into an inanimate object you arent even truly conscious. It's not the same a The Silent Vigil since in that case you aren't transforming into stone, you are stepping into it, there is no change in essential nature taking place (more like a realignment of atomic order to use modern concepts).
For it to be a Greater Maledicition I would suggest it would have to be a matter of a much longer transformation time, not just "until the sun no longer directly shines upon you". Something more to the order of "...and shouldst thou not from sunlight hide, as patient stone shalt thou abide till all thy dearest have aged and died" or somesuch. 100 years as a statue is what I'd call a Greater Malediction. 
Hermetic magic certainly can't change your essential nature, and the Mutp Corpus guidelines specifically state that the spells cannot alter a person's mind. How a Faerie curse will work, is anyone's guess.
Ultimately, Greater Malediction is a Major Flaw and should be balanced against other Major Flaws in the context of a saga. Another Supernatural Major Flaws is Lycanthrope - you change to animal form and mind each full moon. A Hermetic Major Flaw is Restriction - not being able to cast any magic in a storm, on birds, when in a small boat, or so on. Another General Major Flaw is Crippled - you can't walk.
Each of these is a major inconvenience, but isn't directly fatal. Your version would effectively remove the character from the saga, killing it for all practical purposes. I think that's excessive.
I suggest instead that a Restriction (Under direct sunlight) is a bit weak for a Major Flaw by itself, but if combined with complete paralysis (an even larger penalty than Crippled) it makes for a sufficiently debilitating flaw. It is still pretty predictable and unintrusive, but so is Lycanthrope, really - you can prepare for the full moon, and are only affected for a single night out of the whole month.
Fair enough, it was a sleepless night and I was feeling particularly evil lol. Sorry. 
I believe that it is easily a Greater Malediction. Taking Nocturnal for this mage is almost a necessity. Think of these problems:
How does he travel? It's a little harder to see at night (other spells can help this.
How does he attend tribunal?
Most visitors, vendors and transactions take place during the day. Having to make the character do all these things at night would be a pain.
How does he interact with the rest of thr troup? They are all sleeping at night and he is hiding like a sissy girl during the day.
If he only comes out at night, and we all know bad things happen in the nighttime, maybe the grogs or nearby townsfolk believe he is some kind of malevolent creature.
Just restating the broken arm, chopped off head, etc. You don't get your parma against sledgehammer
Not for the fey it isnt.