While I general I'm a little wary of mixing +1/+1 and -1/-1 counters (as it can get very confusing what the size of things arethis, by the way, is where the rule about +1/+1 and -1/-1 counters canceling one another out came from), I do like how this card uses both in a contextual way that gives the card a strong flavor
Getting to deal 20 damage with Nightmare Aura once every turn and on top of that a2-110-darkrai-ex's Dark Prism will put us on 100 damage a turn
However, things get a bit interesting when you play best-of-one matches
But dont be too hasty, opening this pack right away will spell certain doom
Its a powerful thing to do with a lot of mana, but at the end of the day, its not good enough to warrant space in the deck
Note that the instant/sorcery you target has to have an exact equal mana value to the amount of damage your tokens dealt, so it takes some careful maneuvering