I use it whenever there is more than one critter and I have enough of an MP cushion to use it with Dark Arts. I don't use it on single targets, it's just a waste of MP at that point since I can Dark Arts/Carve and Spit. With 4 or more targets, I usually use Dark Arts/Abyssal Drain, followed by Dark Arts/Dark Passenger, since with Blood Price up, I'll get that MP back super fast.