Apparently GMs implemented invisible enemies that SHOULD NOT BE ABLE TO BE KILLED. Using Autokill, Zirak, or Wonderspeed will probably kill them.
You probably will be flagged for a ban if you kill one of these mobs.
Not much is known about the invisible NPC. Some forumers claim that it was always there and that it cannot be killed. I really don't know.
Hack at your own risk!