There is a secret mini-boss named Wicked K who appears at four different locations for you to fight. Every time you fight him, he becomes stronger for the next battle. He will appear at the Choking Grounds after you’ve entered the Drowned Pass, at the Drowned Pass after you’ve entered Anvil’s Ford, at Anvil’s Ford after you’ve entered The Hollow, and at the Twilight Cathedral in an area accessible through use of the Abyssal Chain. You do not need to find him in this order.
Wicked K was added late in development as a joke amongst the developers. While his name is unknown, his textures use the filename “Killington,” so that may be Wicked K’s full name.