It's a card that won't deal a lot of dmg (even if it deals more to rainbows; which is awesome btw). Also, it will take slots in the deck without any chance of gaining card advantage (not even 1:1). And finally, it's darkness, and in darkness you already have a lot of great cards to play. Hard to play this one when you can play Pest, Nightmare, Steal, etc. It's not a reliable kill.
I think it should cost 2|1 to be less painfull to play and more attractive. If it could also heal you in the process, it would be even better. :p