This is a decent card, but not the greatest. I think the main thing holding this card back is its cost. If it didn't cost 4, it wouldn't cost 8 to use its ability the first time. And paying 8 to deal 4 damage to a creature really isn't worth it (Aladdin's Ring anyone?). If this cost 1 or 2, then it'd be a lot more affordable and therefore better. The effect is perfectly costed at 4 since it's 4 colourless mana for 4 damage. The other thing that kind of holds it back is that it can only target creatures, but if it could target players I think it'd be too powerful, especially for just 4 mana.
Pros: 4 damage for 4 mana
Cons: Can only target creatures, requires sac, 8 for the first shot