Blood Drinkers will frequently use their blooddrinking ability which will drain life out of players in range (AoE) and transfer it to the drinker. This is shadow damage and can be absorbed or resisted by the appropriate abilities. They also have an ability that drains 1k life per second from any friendly target and heals them for 5k life a second. This ability will also break sheep, so you need to tank the Drinkers away from other CCd adds.