I used to script a lot of lua back in the arcemu days.
I used the command Unit:SetUInt32Value(58, 26) a lot and I can't find the command for eluna.
This sets the NPC as unattackable. If you click him, he is displayed as hostile, but you won't see the sword when you mouse-over. You won't be able to attack him, at all.
I want to make so the creature can't attack you and you can't attack him.