You see The Blightfather
Notes
It is the Lancer Beetles' boss that spawns with their raid in Muggy Plains.
Abilities
Melee (0-?), Invisibility, makes you Cursed (damage raises up to ?), Poison Bomb (around itself), summons 0-2 Lancer Beetles.
Damage Taken From Elements
Physical 
Death 
Holy 
Ice
Fire
Energy
Earth
Location
Behaviour
The Blightfather will retreat at 4 to 24 (1% - 6%, deep red to red) health.
Strategy
Simply kill it. The Blightfather is as strong as an ordinary Lancer Beetle, just faster.