If I'm understanding how Tainted works, this card is essentially a swing of 12 life (-6 to your opponent, +6 to you) for 16 mana. That's pretty efficient, however, 16 mana is not easily achieved for the Warlock, who generally needs to be using his mana continuously to keep up the pressure. However, there's no dice rolling to do the damage, which is a big plus, and can be a game changer (a swing of 12 life is nothing to laugh at).
This might end up being a better card for other dark mages in the future (like the Necromancer), but the Warlock can make use of it as well, although I don't know how often they will need it, or be able to cast it.