Since nobody has properly explained how to find this boss: after killing the Divine Dragon and being transported to Kuro's room, hop out the window, jump down to the top of the main stairway leading to the castle, and double back through the ground floor. You should be close to the Old Grave idol. Run past it, all the way down to the broken bridge, which has now been repaired. Run across the bridge and you are now in the Outskirts Wall area. Run past or fight the fire drunkard miniboss, and keep running up through the area as if you're going back towards the Dilapidated Temple. Eventually you'll reach a wall of fire with an idol in front of it, use it to be transported to the Flames of Hatred.