How do you do custom particles!! - You can also just change the person into a skull and make some sort of "Consumed into darkness" thing. or Some sort of sludge or ooze to define the poison aspect of the ability ("shadow poison")
I'm using the Alien Swarm Particle Editor. Cyborgmatt once released a simple guide to get it to run. That was actually my first concept for the effect. The problem was that most people would see the skull from top view so the animation would be barely noticeable. That's a good idea! Maybe small, dark puddles left in the…