You see a cart packed with gold.
Notes
It is directly related to the respawn timer of Custodian. 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
Unknown.
Damage Taken From Elements
Physical 
Death 
Holy 
Ice 
Fire
Energy
Earth
Drown
Location
Cobra Bastion basement.
Behaviour
It will walk when there is a player close to it, as long as there are no obstacles in front of it. A cart packed with gold never retreats.
Strategy
Protect it and clear its path to help it collect some gold. Whenever it drops Wheel Oil, quickly use the oil on the Cart to speed it up for a second.