Let's be honest here: this is basically a black Thunderclap (except it's not free). I know it's not exactly the same things, but it essentially serves the same purpose: to kill that little utility creature. Black has better options to kill little utility creatures in my opinion (Murder, Death Stroke, Rend Flesh, Smother, Victim of Night), but this is the only one with flashback, so it gets some points there, despite the high cost. I'd rather pay 6 to kill a Dark Confidant in the late game than not be able to at all.
Pros: Black kill spell with flashback
Cons: Limited targets, expensive flashback