Yow. That's a very powerful black ability. The power level balance seems like it'd be better the other way around. Maybe the life payment slows it down enough, though?

Mmmm... turns all of your black creatures into relatively incompetent assassins, at a cost of 1 life per try.

It's a powerful ability to have sitting around to worry about in the middle of a combat phase though; very flexible.

Is it that powerful? It is when you have a lot of black permanents, of course, but (in most games I've played) you aren't often going to have enough to keep big creatures in check. This is less powerful that Royal Assassin in some ways, especially since it relies on something external to even activate the ability. Admittedly, it is more powerful in other ways.

It seems really good with enchantments, to be honest. Play a black enchantment on rounds 1 and 2, then drop this on four, and you could potentially knock your opponent's game down a peg. Especially if their best creature is a 4/3... they won't be able to get past your 1/3 Cleric.

That said, I don't really know if it's broken powerful. Theros should do a good job telling us how often devotion works... I just haven't played it enough to get a good read. Perhaps Metalcraft gives us some clue as to how often 3 black permanents might be on the table?