May appear as a random boss in a Chalice Dungeon.
The Madman can be tricky boss, since he's basically an NPC Hunter. Use sidesteps to avoid his enchanted sword slashes and blunderbuss blasts, and keep an eye on his left hand. When it's in a tentacle form, he will also use a medium range tentacle strike. He may even perform a large area of effect spell, so keep your distance if you seem him powering up.
Things heat up (quite literally) when the Madman's Escort shows up when the Madman is past half HP. He's another Hunter wielding the Kirkhammer and Flamesprayer. Focus on getting rid of the Madman first, then turn to his buddy. Keep sidestepping the hammer blows, and be sure to give a wide berth when he starts firing the flamethrower your way.