Can't that just be "At the beginning of your upkeep, sacrifice a creature "?

­Demonic Appetite.

This one does allow you to enchant an opponents creature - which might be too good a removal spell, for the loss of +0/+1 when using it on your own stuff.

Huh, true. Hmm.

Went with a 3 CMC since it has no restriction, but if they have more than 1 creatures they at least have one buffed creature to attack with.

Yeah - but now it's probably too good if you can slap it on your own creatures. You've also got a typo there "sacrifice" is written twice.

Tinkered with it a little bit more.

Guh. Buh. DOOOM.

Just to be clear, if I hit this with Chant of the Skifsang, Abyssal Awakening will fall off. I could not think of another spell (singular) that would do that. Though there must be an instant that gives -X/-0...

­Oboro Envoy, Silvergill Douser ... Odd, I can't find an instant that does it, and yeah, you'd really think there'd be one.

Even with such a card, it's not a very common limit. It's probably fair - you're almost certainly giving up at least 2/2 in creatures a turn, considered in that light, this is almost like Unstable Mutation, only very very black.

­Howl from Right Here

(And it's STILL hilarious removal if they don't have many creatures. Hey! Your 15/15? Fancy 21/21? Yeah, let's do that. Your turn. Oh dear...)

Interesting comparison now with Parasitic Implant. That's also a 4-mana aura saying "You sac this creature". The benefit of "and I get a 1/1" vs "and you sac another creature" is probably made up for by the rarity jump.

And on the rare occasions when you ever put this on your own stuff, it's awesome smashtastic Timmyfication.

I don't think you even need the rarity jump - it doesn't force the enchanted creature to be sacced, it's "Opponent sacs two creatures a turn, until this goes away" Giving them that choice probably makes up for the extra.

But yeah, you think smashyness isn't gonna happen often, I think it's smashing! But the amount you pay is fair. This is an interesting card; does a couple of different things, does them fairly well, is relatively expensive for it. I likey.

Vitenka! You can't have a +6/+6 aura at common!

Well, then if it has to be rarer, it probably needs something else. Since there's little fear of dropping two of it in a single turn.

I think the flexibility of working in each of two ways means this'd be a reasonable uncommon.