This + Arsenic = This + Fahrenheit =
Also I don't like the theme at all. I see "Backstab" as something sneaky and precise, like ignoring armor or doing double damage. This card is more like "Flurry" or something.
That could be because at the current moment (with Gargoyle in development) Darkness has the least number of cards in circulation. Or that the general community loves its Darkness.