Micolash, Host of the Nightmare
While ascending Mergo's Loft, you'll reach a strange priest wearing a cage for helmet. This is Micolash, Host of the Nightmare.
This boss fight is different from all the others: it's more of a chase than a battle. The first half takes place in a maze of staircases and bookshelves. Micolash will animate the puppets in the area as you explore, so take a moment to take them out if they spawn near you. When you spot Micolash, chase him down until he reaches a dead-end room. In here you'll be able to whale away half his health bar in no time. Just watch out for his midrange tentacle attack or punches - both can be avoided with simple dodges.
After reaching down half health, Micolash will teleport away. Head back to the staircases, out the new exit to the right, and up the spiral staircase outside to reach a second maze. Chase Micolash again - he'll drop off a few ledges this time - until he locks himself behind a large gate. Now you have to head to the top of this area and look for a small passageway that'll drop you into the room with him. When you get there, use the same attack strategy as before, but watch out for this new spell attack: orbs will burst all around the room, so use a dodge roll as best as you can to avoid the heavy damage. Chip away his remaining health to end the madness and win the fight.