Note: It is a pseudo insta-kill, like Freeze or Basilisk Blood, because it makes the creature unable to attack or use abilities. It is weaker than Freeze and Basilisk Blood because it requires a two-card combo (Mark + the damaging card) and because it is less effective against Mitosis, Butterfly Effect, etc. It is stronger than Freeze and Basilisk Blood because it lasts indefinitely (as opposed to 3 turns and 6 turns, respectively). Compared to a true insta-kill, like Freeze+Shockwave, Mark is more effective than Freeze because the death triggers for Mark (Fire Shield, Thunderstorm, Plague, etc.) are more numerous, are probably more useful outside of the combo, and are also easier to proc (AoE vs. single-target) than the death trigger for Freeze (Shockwave). It is weaker than Freeze because Mark by itself does not affect the creature.
Conclusion: I would argue that this card is a bit more powerful than it should be, and thus, I would increase the cost to 3/2, or add a duration for the ATK reduction (10 turns would be more than short enough).
