Making Ghosts?

on my server i have a big tomb and i want to put ghosts in there, basically floating swords. i been messing around and the best i can get is if u hit the mob it will go invis but i want it to be invisible at spawn, the second it spawns its invisible. Anyone know how to do it?

The skill

- potion {type=INVISIBILITY;duration=999999;level=8} @Self ~onTimer:1