Played with this in the draft as -2/-2 to one thing and -1/-1 to another, and it was crushingly powerful. Then Scars of Mirrodin printed Arc Trail, which Tom LaPille called "crushingly powerful", and that's uncommon. This needs to change rarity or text. Perhaps two lots of -1/-1 distributed among 1 or 2 target creatures.

Templated as per Seeds of Strength.

Arc trail is crushingly powerful, in limited however its really harder for people to play a BB spell than a 1R spell, the difference can be really big, I'd say you could probably make this an instant for 2B or maybe change the cost from BB to 1B and keep it as a sorcery, as -1/-1 effects are generally better removal than damage solely because of the combat tricks you can pull when you add attacking creatures to the mix, but you take that away when you make it a sorcery, for BB sorceyr i'd say you could even go as far as to give -2/-2 to one and -1/-1 to another.

It used to be -2/-2 to one and -1/-1 to another. That was the version that was crazily powerful, even as a sorcery. This needs to be a sorcery because black is the primary sorcery colour in this set. It's not a combat trick, sure, but it was painfully good removal in limited, particularly since the set has a lot of x/1s and x/2s. So I don't think I'm inclined to put its power back up.