You see a charged energy elemental.
Notes
They are found in a knight-only accessible area (accessed from the Ivory Towers in Edron), after the Summer Update 2009 they are weak to Physical Damage.
Ops, too much lured...
Abilities
Damage Taken From Elements
Physical 
Death
Holy
Ice
Fire
Energy
Earth
Drown
Location
Behaviour
A charged energy elemental attacks in melee range. A charged energy elemental never retreats.
Strategy
A knight of level 80 or higher should have no problem killing them. When they come in a larger pack of creatures you could use Whirlwind Throw to speed up killing and Berserk or Fierce Berserk when more elementals are lured. Remember to bring food, because they don't drop any food.