So that's a guaranteed EM for your opponent if he wins... HP left : 1. Max HP : 1. He finished you with all his HP's !
I suggest that you replace "Every turn" by "On each successful attack" on the card text (NB: an attack with strictly negative attack is considered succesful). Let me explain why:
If you use this as your only source of offense, you really don't care about the negative-attack healing effect. You have a creature dealing 2 "damages" (3 when upped) *every turn*, ignoring all shields, Sundial, and preventing the opponent from healing. In short, the *only* counter to this card is CC.
Apart from that, I really like the card, and I'm waiting from more of the Void/Divinity series !
With Mark of Aether, it could go like this:
(Turn 1) 4xAether Tower, Void Stalker. Opponent's max HP: 100-3=97
(Turn 2) 2xVoidstalker, Fractal, 8xVoidstalker. Opponent's max HP: 97-11*3=64
(Turn 3) Nothing. Opponent's max HP: 64-11*3=31
(Turn 4) Nothing. You won.
Voidstalker has 4hp, enough to resist a firestorm. If your opponent controls just a few of them, that would only delay your victory of one or two turns. The only counters would be 1) a lucky pandemonium 2) TWO firestorms and 3) Outrush you thanks to the extra quanta
Fractal/Voidstalker would definitely be OP. However, I don't think Voidstalker is OP, I think Fractal is. But this is another story.