Duke Bovicus

Not much is known about Duke Bovicus except that he is responsible for the rise of Hellcows in the west.

Quests/Interactions
He can be summoned from the portal to Hell at Hellstrom Ranch by hitting the cowbell with an Infernal mallet. This will cause the player character to be instantly knocked out unless the ground has been sprinkled with cowsbane.

If the ground has been sprinkled with cowsbane, Duke Bovicus will be trapped and his reign of terror ended. This is the final step in the Hellcows side quest.

It is possible to interact with him to either free him (which will still lead you being able to trap him again), or accept a pact, which affects your stat (but does not alter the consequence at the ending.), but can be chosen not to free later.

Location
Hellstrom Ranch, just outside portal to Hell.