Hmm. I like the rest of this cycle, but Nekrataal and similar effects (Keening Banshee, Morkrut Banshee, Flametongue Kavu, Murderous Redcap) are uncommon for a reason. At common this will show up with incredible regularity in Limited, almost certainly more often than you want. It's a very strong version of the effect too: this will 2-for-1 all your coloured fatties all day long.

I'd recommend dropping the cost and making the effect "gets -1/-1 UEOT", like Blister Beetle, Blisterstick Shaman or Sparkmage Apprentice; that's the version of this effect that seems to exist at common these days.

Man, I remember when Brainspoil was a strong card because it could kill almost anything, yet it only cost . Murder has kind of changed the way we think about that, though, I suppose. Ignoring that, in a slow draft environment, I would still draft this card as fast and as often as I could. Kill target creature is still pretty good at .

I don't know if there is no room for nekrataals in limited, like Alex suggested. Before Æther Adept became a core set standard, we hand't seen many straight up Man-O-Wars for a long time. Sure it's rare, but it doesn't really break anything... but maybe it should cost +... I'm not sure if you want it to be limited MVP...

Actually, it said "destroy target non-artifact creature" earlier, so this isn't necessarily worse...

I guess this is broadly comparable to Hornet Harasser, at common. This is much better at getting you the effect when you want it, and easier to cast, but does need one more mana. Probably fine now.