I'm not high on this card at all. "Sometimes" counter a spell? "Sometimes" kill a dork? And the opponent can control when the condition is satisfied?
In the early game, when cheap, targeted removal or countermagic is at its most powerful, this card is most likely to miss. And, if your opponent knows you have this, they can plan around it. As the game goes long, this is a decent enough spell, but then it starts to have competition.
Ultimately, this feels like another card kind of like Dimir Charm or similar effects. On the edge, not there.