I am in full favor of these troops. They aren't overpowered, as referenced by the hitpoints for each level. The damage rate is excellent. I liked the touch with the Inferno Tower weakness
Im really unsure of this idea, atleast it's not unreasonably OP. But there should be a speciality with this like the other dark troops. Maybe allow the dark assassin to be invisible at the brink of death for a few seconds. Just a thought