Taking four Prizes off a Mega... yikes.
Of course, this doesn't automatically make it good. For one thing, it takes three Energy, meaning there's no real way to charge it up in one turn in Standard (Expanded gives you Dark Patch). For another, it only does 70 damage, and pretty much every Mega has at least 3x that much HP; the only one that doesn't, M Diancie-EX (190 HP), happens to resist Darkness anyway.
Needless to say, this guy won't be the center of his own deck. However, I can see it being used as a 1 or 2-copy tech in Yveltal decks, especially since Yveltal-EX can easily bring a Mega down to KO range for End Game.