I kept this long time a secret until now.
There is a questitem called Scepter of Domination for the quest Betrayal
It is supposed to give you control of a single NPC there, but on my server I was able to control every NPC and use their buffs and spells.
For example you can control a boss in an instance and go solo everything else with him (Patchwerk is my favorite enrage+hateful = over 50k hits :-).
The control breaks after 2 to 3 mins and the npc will aggro you.
The channeltime of the scepter is 3 sec so if you charmed a really strong npc that would 2hit you, you'll probably die.
My tip is to dismiss your minion in every fight longer than 5 secs and re-control him.
If used on players, it will bug them totally out. they are unable to target anything and everytime they use a spell it says "can't do while charmed"
Only tested on one server, dunno if it works for you.
Good luck and have fun