Boss: Micolash, Host of Nightmares
Micolash is the most cowardly boss you’ll find in Bloodborne, running away from you until he is cornered in the maze-like library. The fight takes place in two stages: an encounter on the first floor when he is cornered that will end when he reaches below 50% and another after you chase him through the upper part of the library. Once you do find him, focus on keeping him in sight rather than trying to attack him.
After the cutscene you’ll need to find Micolash in the small lower section of the library, which shouldn’t be too hard. Follow him around until he enters a hallway with multiple arches and then begin your attack.
Focus on taking out the two Skeletal Puppets first and then focus your attention on Micolash. The fight should be relatively easy, provided you do not try and run straight at him resulting in being hit with his Augur of Ebrietas, and when he reaches below 50% he will pretend to die and teleport to the upper section of the library.
From the room you fought Micolash take a right up the stairs and head through the now open archway to the main staircase. There are two Mergo’s Attendants: once with a flail and another with a crossbow on the staircase that you’ll need to take out before you enter the upper library.
Head up the stairs to run into Micolash, who will jump through mirrors, and then backtrack and take a right to drop from the balcony into the room where he exited. At this point he may run through another room with a mirror or run into a room and lock the gate behind him, blocking you from him. Either way, once he is locked behind the gate you’ll need to track him down.
Take the staircase to the left of the gate and turn left twice, following the stairs up to reach a room overlooking the large bridge below. From here, look for a gap in the railing on the right to find a hidden path that will drop you into the room with Micolash.
In the second fight with Micolash he will be highly aggressive and uses the A Call Beyond spell if you give him too much room to breath. It is advised that you stick close and circle him, baiting him to use his Augur of Ebrietas attack, and then get behind him with a quick weapon to use a full charge attack followed by a visceral attack.
Your main goal for this fight should be to keep him from casting the A Call Beyond, which will often one shot you, as the rest of his attacks do negligible damage.
Drops: 48,400 Blood Echoes and the Mensis Cage
With the fight complete you can now check the library areas for the following items: Quicksilver Bullets x6 , Frenzied Coldblood (9) , Blood Stone Chunk , Kin Coldblood (10) , the Iron Door Key , Blood Vial x8 , Sedative x3 , the Moon Rune, and Quicksilver Bullets x6 .
You may also want to run through here after refreshing the area to kill some Wandering Nightmares.