The Yahar’gul Chapel is another required location in the story right after defeating Rom.
Transport to the Yahar’gul Chapel by teleporting through the lanterns at the hunter’s Dream and proceed to Yahar’gul. Another route you could take is by heading to the Grand Cathedral and proceed downwards at the right of the Grand Cathedral staircase.
You may see a Lesser amygdala on the staircase and enter to the now opened door. As you enter, go straight ahead and down the stairs. Get the Forbidden Coldblood on the side. You may be attacked by some Afflicted Yharnamites here. Go further down the stairs and get a Blood Stone Chunk from a corpse on the side. Take another step downwards to an atrium where you can get Iron Yahar’gul Item form a corpse in the middle.
There will be an elevator room on the side but it won’t be available for use at the moment yet.
Go back up and take the other stairs downwards where a mob of Afllicted Yharnamites are waiting and where amygdala's vortex also waits. Run away from them and enter the building. Once you’ve entered, kill the Summoner at the second level of the building. Inside one of the cells inside the building, you will get Bolt Damp Blood Gem from a corpse.
Enter the door on the side and go down then go through the opening from the side wall. More Ogres and Afflicted Yharnamites will be summoned here so go straight and right and get the Heir Rune from a corpse sitting on a chair. Proceed through the prison cells and kill the Summoner on the upper platform right beside the Gattling-Gun-Wielding Old Man. Open the door behind you too. Go inside the prison cell and get Frenzied Coldblood from the corpse inside.
Head down to the quadrangle and go down the stairs and turn right where a skeleton sitting on a chair is is - there will be Hags waiting around.
Back up and go up the stairs in front of where you came from and get Blood Stone Chunk from the corpse at the end of the pool. By this time, the Hooded Hags will be attacking you together at the same time. amygdala will also be shooting lasers at you so prepare to dodge as fast as you can.
Head straight down as soon as amygdala fires and enter the building and light the lantern.
Turn left and go to where a skeleton will be sitting on a chair and go down and get Frenzied Coldblood from one of the corpse on the rooftop. Continue on down into the courtyard and defeat the Ogre, Shooting Afflicted Yharnamites, and two Rabid Dogs. Go down the next stairs and get a Blood Vial from a corpse.
Turn right into the bridge way and open the door--this will lead you back to the place in Yahar’gul Village (Herad God).
At the Atrium, there will be a hunter waiting that will give you Clawmark after defeating it. The other two hunters who will attack you will grant you a Threaded Cane and Rifle Spear Hunter respectively along with some Beast Claw Weapons. You may also pick up some Madman’s Knowledge from the dead hunters.
After the battle head out into Yahar’gul village and head down towards the Gate which was closed before.
As you go down into the prison cells you may battle several Human-Leg WereWolves. You may notice that several corpses of Abductors are around which will also give you Blood Stone Shards.
At the other side you may battle a bunch of Skeleton-Headed Spiders which may give you Damp Blood Gem.
Go down to the right and into the place where the gate was locked before and where two carriages are waiting. As you enter, there will be more abandoned carriages and wagons in the street. There is also an Afflicted Yharnamite Shooting and a Giant Spider inside a carriage.
Head straight and climb the bridge on the side to kill the shooter. Get Blood Vial and Frenzied Coldblood from the corpse. Cross the bridge and enter the room at the end where you will find a small glowing bath. Inspecting the Bath will transport you to Yahar’gul.
On the side there is a bridgeway leading outside where you can kill the witch Summoner.
From the bridge earlier, head down the stairs on the side of the glowing bath room. One side will lead you to a Werewolf with Humans legs that will give you Tempering Blood Gem Stone once it is killed. There will also be a tunnel that will lead you back outside.
After the Bath transport, go out and turn right and down the stairs. There will be a lot of Rabid Dogs waiting along with Leg-faced Werewolf. You may pick up the Blood Stone Chunk from the corpse and Blood Stone Shards from the enemies.
Go back and head back out in the street by going down the ladder. When you go down you may pick up Madman's Knowledge. Go straight into the arched entrance and into the plaza facing the Blood Moon where a cut scene will commence showing several Witch Summoners.
Upon entering this atrium, this is where you will battle The One Reborn. Once you hit it in the middle for several times the beast may get stunned for a moment.
During the battle there will be tons of fire bolts thrown at you. Hitting it at the tail seems to be most effective. The One may also unleash an orb around it which may stun you. Getting too close or staying too long inside will make The One stomp around you.
You may also kill the Summoners by going at the side of plaza.
Once you’ve beaten The One Reborn, you will get the Yellow Backbone. Light up the lantern afterwards.
After the battle you may head straight to the stairs and into the building where you will find a mummy in the middle of the room. Inspecting it will lead you to the lecture building