So the way this card is powerful currently feels like it is because it gives you all those souls due to Portal and Harbinger stacking so much life lost in one turn.
All that needs to happen to balance it, is to make it so only damage coming from Harbinger counts towards the Souls. Simple. Then it won't be as powerful.
It could even be balanced to give a flat amount of souls, like 2 or something, so that if the requirement of tainted isn't met, it will still give some benefit just from playing it. But the cost should also increase to 4 mana. Because unlike Portal the value from this card alone is pretty high, and the downside is easier to manage.