The HP drop is the real kicker. With a couple of these and an owls eye you could cut down almost anything in a couple turns.
It also makes a very good counter to GP. Armagios go from 25 hp -> 12 hp -> 6 hp with just 2 of these.
One question, if you hit a voodoo doll with this, will it inflict corresponding damage upon the opponent? ... That could be extremely potent.