Pretty cool. I'll add it to the list, if you don't mind.

By all means. It's here for people to play with.

I'm a bit unsure how it's meant to work mechanically, though. Normally, you need to decide a spell's power before you know all of the factors that affect the size of the power bonus from this. Like, if I throw up a wall of unlight to stop attacks, I don't know whether it's gonna be blocking fire and light or not. So can I call up the extra shifts of power or not?

Not 100% sure I'm grasping the question, but the power bonuses are always present, can only be used to increase the attack or block strength (not area or duration), and are only applied to negate part of a superior opposing roll. Quick and dirty: if a Unlight augmented attack of +8 hits a fire Block of +10, the +4 bonus serves to overcome the difference (the excess is ignored), allowing the Weapon: 8 attack to affect the target normally. The attack wouldn't get this bonus against a spell Block defined as, say, a wall of ice. In the reverse situation, the extra shifts can only be used to affect Block strength, and only come into play against an attack of the prescribed effect. I'm not sure why I'd need to know the opposing effect beforehand.

In retrospect, it does occur to me that quibbling over what type of "matter" is being affected (Nevernever or Corporeal) is probably needless over-complication.

Also, is the Immunity-degrading thing meant to be in addition to the power bonus?

It is, but it ONLY serves to downgrade Immunity; other Toughness powers are unaffected, barring the attack satisfying a Catch.

Hope that makes more sense, and in any case, feel free to (no pun intended?) butcher it at will.

You could get REALLY wicked and have the fae a) put the bargaining player in a 24 hour stasis, and b) have the fae shapechange/glamor themselves AS the player, and "adventure" with the rest of the group for "one day." Havoc ensues... particularly if you give the player the Fae's character sheet, and collaborate with them on what it's agenda is.

I've GMed for a long time. I have had many things thrown at me, in that capacity... 

I'm of two minds about this one. The Laws I'm putting together here I'm largely trying to keep as a "there will be some kind of direct backlash/side effect caused by Magic itself" rather than "This is an arsehole move, don't do it" (those largely come under mundane laws), and I can't recall something talking about a direct feedback to the caster from shapechanging magic, unlike killing and mind screwing, which we have some very direct results of.

I'll need to ponder that one some more.

If your game, and your as-yet-unnamed governing body, make the default White Council assumption about transformation, then the target is effectively destroyed. Their human body and mind are irrevocably gone, in many cases, especially if they remain in their transformed state too long. This period of time can be "instantly", if the transformee is weak-minded.

Also, said governing body would be the ONLY ones capable of policing such activity, unless there's actual "real world" precedent there for people telling a court "She turned him into a newt!" and being believed.

In short, if your "magic police" have extensive back-history (and it sounds like they do), they'd have to have created policy to cover such things.

/2¢

I'm not entirely sure why the power was taken, really. The character is a "psychic biker", and for a long time was played as though she didn't believe in the supernatural, at least beyond psychic visions and psychometry. It did strain credulity after a while, and the character has finally been convinced that monsters and magic are real.

Then it might be time for an update to the character at the next viable milestone. Granted, it's a big no-no to change a character's high concept or trouble aspects, but this seems to be more a case of an overdue retcon of an ability for which the group as the whole didn't fully understand the ramifications. It might even be worthwhile to speak to the player beforehand and discuss the idea of taking on an Extreme consequence (or a very nasty concession) to make losing the ability part of the story. A lot of epic story potential, there.

I'm starting to think Cassandra's Tears is one of those powers that players just shouldn't be allowed to take.

I tend to agree with this just on general principal. At the very least, something like this shouldn't be taken unless all the players in the group agree that they can accept the full nature of the ability and run with it.

Another suggestion has been to have a vision start off incredibly cryptic, but as the story goes on it recurs and becomes more and more clear. I'm not sure how to pull that off, again given that we play for 2-3 hours a week.

Having run games with this type of power, you're correct: that's not always possible. My own method frequently means making the vision cryptic enough that there's no obvious way for a group to avoid it, because it contains too little specific information to know what it's even about. It's also extremely important to design the vision in such a way that the efforts taken to avoid the apparent outcome actually cause it to occur. In short, make the vision look like a disaster, and then make player group victory look like the vision. Granted, it's also a good idea to make player group failure look like the vision, but hopefully you won't have to cross that bridge often.

My tl;dr way of saying I agree with Taran. 

The funny thing is, that none of the visions so far have actually come true, because the group has always acted to prevent them. We only have about one vision per story arc, but I'm still not sure how to handle it. Anything I could do, aside from insisting that the players RP it, seems either over-powering or disruptive.

Then yes, I'd guess the visions are much too specific. They shouldn't have any idea how to prevent it, even down to the wire. The trick, as mentioned above, it to make the outcome look like success AND failure, as dictated by the actual outcome. It shouldn't be showing them the actual outcome, it should be showing them the setting that the events occur in. I hope that makes sense; it's not an easy thing to describe, and I've done it a lot in some games. The last year (of three) of my weekly Legends of the Five Rings campaign was rife with this kind of thing. Keeping six players headed toward the same event, while all being guided by six radically different visions was a tremendous amout of extremely rewarding work.

In most circumstances, I'd go with a compel, too. But I've had players, one in particular (the husband of the person playing the psychic), explicitly state they're not comfortable trying to RP not believing the visions.

That's going to be a very touchy point, and probably the biggest justification for retconning the power out of the group. I don't know any of the people involved, but my hindbrain is warning me that he's having difficulty in 1) separating "truth" from "fact", in context of the game, and more importantly 2) having difficulty separating the RP of a) disbelief in what his wife tells him, b) disbelief in what his wife's character tells his character and c) disbelief in the information given by the visions.

It's just not worth the potential strife it can cause.

Looking at Cassandra's Tears (YS172) and the section on Predictions and Prayers (YS324), the power itself does two things. It a) places an Aspect on the campaign, and b) it gives the player a -2 to convince anyone that the aspect is present (that the vision is true). My own take would be that the -2 really only affects NPCs. It seems a bit odd to make such rolls between players, unless they're prone to internal Social conflicts. However, is also creates an Aspect that gets placed on the entire campaign world, and lasts until the events in the vision come to pass. I'd simply present a Compel to any fellow Player she wants to tell about it. If they accept the Fate point, they don't believe it. Player ownership of this allows them to describe how they react; derisive laughter, polite dismissal... whatever. Since I'd always tend to use the power to signify that Something Really Bad™ is about to happen, they're all likely to want the Fate point later, anyway. And keep in mind, nothing prevents the player with the visions from pursuing her own agenda, investigating the vision, and potentially dragging one or more other players along, particularly if they have bonds (Aspects) of group loyalty, or somesuch.