You see a soul scourge.
Notes
It's part of the Grave Danger Quest and assists Duke Krule
Abilities
Melee (?)Damage Taken From Elements
Physical 
Death 
Holy 
Ice
Fire
Energy
Earth
Drown
Location
Under Thais Ancient Temple
Behaviour
A soul scourge attacks in melee range. A soul scourge never retreats.
Strategy
You can kill them with area attacks while attacking the boss but they should't be your main focus.