You see a force field.
Notes
Part of The Secret Library Quest.
Abilities
Damage Reflection (10-20%)Damage Taken From Elements
Physical 
Death 
Holy 
Ice
Fire
Energy
Earth
Location
Lokathmor's room inside The Secret Library.
Behaviour
It doesn't move.
Strategy
Destroy one of them as quickly as possible to leave the trap. Note that due to their Damage Reflection it's recommended that single target attacks are used. Mages should specially avoid using Ultimate spells at all costs as this can instantly kill a level 600 if all Force Fields of the room are hit.