You see Custodian.
Notes
It's an Order of the Cobra boss, and part of the Grave Danger Quest.
Its respawn time is directly related to the Cart Packed with Gold. The impact is confirmed via community tests, but the specifics are still unknown: it seems the cart reaching end of the tunnel or being destroyed is a prerequisite for the respawn timer of Custodian to start counting.
Abilities
Melee (0-500+)Damage Taken From Elements
Physical 
Death 
Holy 
Ice
Fire
Energy
Earth
Location
Behaviour
Unknown