well, as ive said im not too high a level.
but this is how it worked on a HUGE orc (forget the blue guy's name...)
cast it, and he is surrounded by a magical cage. i dont know if he could hit me if i went close, but as an archer it kept him immobile while i fired arrows at him. so a mob i usually have to kite for a good couple of minutes went down in less than 45 second (with no danger to myself)
it is doesnt work (i assume they resisted) the cage appears, they walk right through it, and the cage goes away. I havent been able to cast it on undead so far, but not sure if that's a lvl of mob resistance, or if all undead can walk through it. which would make sense, it would be silly to think a ghost could be held in any kind of cage, energy or not.