Hmm... I think I see where you are going with this... but the wording is going to be tricky I think.
Right now, your wording would imply that you would NOT want to use it with nightmare. Otherwise, the opponent would discard it to gain 15 hp of healing.
It would work quite well with fractal though.
I want it like that. It gives nightmare decks a way to destroy sosac.
Hmm... I think I see where you're going on that... but the problem is, they could simply wait till SoSac wheres off, pop a dial or dim shield (which many Sac decks have) and then start discarding these to rebuild HP while they wait to find their next shard.
Also, some SoSac decks make use of light quanta... so giving them a nice creature to use against you may not be the best idea.
One odd idea I had on this though... What if you have the ability heal the player with the lowest HP.
After popping the shard, the opponent would likely have the lowest HP, making them vulnerable to this. All you would have to do is fractal this and then discard it.
Works against SoSac much better that way I think. And it will be useful for that whether you spam it with fractal or nightmare, and also sets it apart from other more general purpose heals.