It increases the amount of damage you absorb the more you level it. I think mine is at 55 and absorbs 12.5% of everyone's HP for 15 sec.
Ex. Me at 60k HP it absorbs around 7k damage but if the tank is at 100k HP its absorbing 12.5k damage.
not too op but helpful if everyone just dropped in health and gives a little buffer till the healer gets everyone's health up. My real use for it is in siege but I'm not gonna devulge my secrets