I really like the idea of this card, as it expands the game in a new direction that we don't really have yet, other than something like ghost of the past. The only thing I don't like about it is that you'll almost never actually pay the activation cost, or the casting cost for that matter. Maybe there should be some sort of advantage for hard-casting it, or disadvantage to playing it from hand (comes into play with half hp from hand, maybe?)
Also, this isn't as good with fractal as you might think, at least not really any more so than with other good creatures. Sure, you've got a bunch of these in hand. Then, the next time you take lethal damage, they ALL come into play at once, but only the last one actually has gravity pull, so you get at most 8 damage worth of attackers, although with eclipse/nightfall that obviously increases.
Thematically, I think the card is fine in darkness; you duck into the shadows to avoid the deathblow (and leave your minion to take the hit :p) and of course the gravity activation cost makes more thematic sense over any other element.