I think I'd recommend removing the delay and dropping the bonus down to 1/1 or 2/2. You could still get the eclipse benefit. Also note the unlikely synergy with holy flash :p
In its current version, though, this card is too strong. It's cheap, and +5/5 is a lot. Beyond that, you can even use it as creature control, kind of like a 1 turn basilisk blood (in an emergency) or combo it with holy flash, though admittedly those two uses won't be very popular. The point is, though, that the card is both versatile and powerful, probably too much so. Another thing you could do is play several of these on the same creature in one turn, and the creature would only be delayed one turn. (the way that delay currently works right now means this card would behave that way) You could get around that by making it target only non-darkness creatures, as others have mentioned.