You see a regenerating mass
Notes
It's Nargol the Impaler's regenerating mass during the Grave Danger Quest.
Abilities
Melee (0-?)Damage Taken From Elements
Physical 
Death 
Holy 
Ice
Fire
Energy
Earth
Location
Behaviour
Fights in close combat. A regenerating mass never retreats.
Strategy
Trap the location where it spawns before hand and then deal as much damage as possible, you only have 30 seconds to kill it or the boss will respawn.