Pthumeru Ihyll Bloodborne Wiki » Chalice Dungeons » Labyrinths » Pthumeru Ihyll

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Table of Contents

Gallery Page

Basic Information

Flow Chart

General Notes

Enemies

Item Treasures

Walkthrough & Map

Basic Information

New Players ► Recommended: Player Level 120+ & Weapon Upgrade +10

Labyrinth	Chalice	Required Materials	Layer Bosses	Key Rewards
Pthumeru Ihyll Depth 5		Ritual Blood(5) x9 Red Jelly x4 Arcane Haze x25 Living String x1 Blood Echoes x11500	L1: Pthumerian Descendant L2: Bloodletting Beast (Parasite) L3: Yharnam, Pthumerian Queen	L1 Boss: Tempering Damp Blood Gem(6) L2 Boss: Pthumeru Ihyll Root Chalice L3 Bath Messenger Shop: Sinister Pthumeru Ihyll Root Chalice L3 Boss: Yharnam Stone

Pthumerian Flow Chart

Sinister Chalices										Pthumeru The underground labyrinth is extremely large, and changes its form the deeper it goes. Each part has its own unique appearance and tendencies, though, and these are divided into four distinct areas: Pthumeru, Hintertomb, Loran and Isz. Pthumeru specifics The standard area type, usually with a basic room structure. No particular elemental type dominates. Kidnappers appear often. Merciless Watchers guard floor levers.
↑					↑				↑
Preset Chalices		→		→		→		→	YOU ARE HERE
↓	↓		↓		↓		↓		↓
Root Chalices

General Notes Two of the four Red Jellies you are going to need to help craft this dungeon can be found in a chest in the Lecture Building, and the other two can be found in the Lower Loran dungeon. You can get the required Arcane Hazes from the first Pthumerian Labyrinth from the Fluorescent Flowers on the third level garden plaza area, or use the Workshop Arcane Haze Extractor to extract them from any surplus basic ritual materials you can spare. The Living String is the hardest ritual item to get. You have to seek out and defeat the hidden Brain of Mensis in the Nightmare of Mensis area. As with all Depth five Chalice Dungeons, it is advised and essentially more fun to take this on in an online co-op team. Note: There's a hidden chest in the wall, in circular elevator room top floor on 2nd layer of Pthumeru Ihyll. It contains a red jelly. [video] Old Hunter Allies: With patch v1.07 a wide variety of new and existing NPCs have been added to the main game, the Old Hunters areas and the Chalice Dungeons to aid you during the hunt, particularly during boss fights. The conditions for summoning these allies vary; the “Old Hunter Bell” is a universal requirement for calling any of them. In some areas you may be able to summon up to two NPC Hunters. In Pthumeru Ihyll you can summon Tomb Prospector Olek on Layer 3 for Yharnam, Pthumerian Queen boss, his sign is in the ladder well room with the locked exit door. Look to your left as you enter. Summoning him will forbid you from summoning Queen Killer on this floor. You can also summon Old Hunter Queen Killer on Layer 3 for Yharnam, Pthumerian Queen boss, his sign is in the hallway, a little to your left, just before you head down the stairs to the boss room. You can also summon Madman Wallar on Layer 3 for Yharnam, Pthumerian Queen boss, once you've made your exit from the first tunnel, you can summon Madman Wallar from behind the large tombstone directly ahead. You won't be able to enlist the help of Prospector Olek and Queen Killer at the same time, but Madman Wallar can still be summoned regardless of who you choose, so be sure to summon him if you need the extra help.

Enemies

Layer 1			Drop
Regular Enemies
Labyrinth Watcher (Dagger)	944	5801	Blood Vial x1 [8%]
Labyrinth Watcher (Twin Axes)	1483	5801	Blood Vial x2 [8%]
Labyrinth Watcher (Cleaver)	944	5801	Blood Vial x1 [8%]
Labyrinth Watcher (Halberd)	1483	5801	Blood Vial x2 [14%]
Watcher's Gravedigger (Rifle, Hook & Lantern)	1752	7360	Quicksilver Bullets x3 [100%]
Watcher's Gravedigger (Hook & Lantern)	1887	7273	Blood Vial x3 [10%], Blood Vial x2 [30%]
Labyrinth Rat	944	4589	Tomb Mold(5) x2 [0.4%], Tomb Mold(5) x1 [4%]
Nightmare Apostle	984	4936	Quicksilver Bullets x2 [30%]
Rabid Dog	1078	7273	None
Hunting Dog	1213	8659	Twin Blood Stone Shards x1 [30%]
Strong Enemies
Merciless Watcher (Scattergun, Club & Lantern)	4245	27362	Random gem based on area x1 [25%], Blood Vial x3 [30%], Quicksilver Bullets x4 [25%], Ritual Blood(5) x1 [20%]
Merciless Watcher (Saw)	4245	27362	Random gem based on area x1 [25%], Blood Vial x3 [30%], Quicksilver Bullets x4 [25%], Ritual Blood(5) x1 [20%]
Watcher Chieftain (Club & Lantern)	4245	27362	Random gem based on area x1 [25%], Blood Vial x3 [30%], Quicksilver Bullets x4 [25%], Ritual Blood(5) x1 [20%]
Bloodlicker	3100	18270	Random gun type gem x1 [16%]
Evil Labyrinth Spirit	1415	9698	Random gem based on area x1 [8%]
Hunter Enemies
Nameless Tomb Prospector (Kirkhammer & Repeating Pistol)	3468	26096	Random gem based on area x1 [20%]

Layer 2			Drop
Regular Enemies
Labyrinth Watcher (Dagger)	991	6092	Blood Vial x1 [8%]
Labyrinth Watcher (Twin Axes)	1557	6092	Blood Vial x2 [8%]
Labyrinth Watcher (Cleaver)	991	6092	Blood Vial x1 [8%]
Labyrinth Watcher (Halberd)	1557	6092	Blood Vial x2 [14%]
Watcher's Gravedigger (Rifle, Hook & Lantern)	1840	7728	Quicksilver Bullets x3 [100%]
Watcher's Gravedigger (Hook & Lantern)	1981	7637	Blood Vial x3 [10%], Blood Vial x2 [30%]
Labyrinth Rat	991	4819	Tomb Mold(5) x2 [0.4%], Tomb Mold(5) x1 [4%]
Nightmare Apostle	1033	5182	Quicksilver Bullets x2 [30%]
Rabid Dog	1132	7637	None
Watcher's Gravedigger (Pickaxe)	1981	7637	Blood Vial x2 [29%], Tomb Mold(5) x2 [0.7%], Tomb Mold(5) x1 [8%]
Wandering Nightmare	382	1273	Random gem based on area x1 [100%]
Lost Child of Antiquity	2264	11365	Random Nourishing gem based on area x1 [8%], Twin Blood Stone Shards x1 [18%], Blood Stone Chunk x1 [1.5%]
Labyrinth Ritekeeper	1274	7092	Random gun type gem x1 [2%], Ritual Blood(5) x1 [4%]
Gravekeeper Scorpion	637	5364	Random Dirty gem based on area x1 [4%]
Strong Enemies
Merciless Watcher (Saw)	4457	28730	Random gem based on area x1 [25%], Blood Vial x3 [30%], Quicksilver Bullets x4 [25%], Ritual Blood(5) x1 [20%]
Watcher Chieftain (Club & Lantern)	4457	28730	Random gem based on area x1 [25%], Blood Vial x3 [30%], Quicksilver Bullets x4 [25%], Ritual Blood(5) x1 [20%]
Bloodlicker	3255	19184	Random gun type gem x1 [16%]
Merciless Watcher (Mace)	4457	28730	Random gem based on area x1 [25%], Blood Vial x3 [30%], Quicksilver Bullets x4 [25%], Ritual Blood(5) x1 [20%]
Labyrinth Warrior (Crossbow & Sword)	2689	28094	Random gem based on area x1 [25%], Blood Vial x4 [15%], Blood Vial x3 [45%], Yellow Backbone x1 [15%]

Layer 3			Drop
Regular Enemies
Labyrinth Watcher (Dagger)	1014	6382	Blood Vial x1 [8%]
Labyrinth Watcher (Twin Axes)	1594	6382	Blood Vial x2 [8%]
Labyrinth Watcher (Cleaver)	1014	6382	Blood Vial x1 [8%]
Labyrinth Watcher (Halberd)	1594	6382	Blood Vial x2 [14%]
Watcher's Gravedigger (Rifle, Hook & Lantern)	1884	8096	Quicksilver Bullets x3 [100%]
Watcher's Gravedigger (Hook & Lantern)	2028	8001	Blood Vial x3 [10%], Blood Vial x2 [30%]
Labyrinth Rat	1014	5048	Tomb Mold(5) x2 [0.4%], Tomb Mold(5) x1 [4%]
Rabid Dog	1159	8001	None
Watcher's Gravedigger (Pickaxe)	2028	8001	Blood Vial x2 [29%], Tomb Mold(5) x2 [0.7%], Tomb Mold(5) x1 [8%]
Wandering Nightmare	392	1333	Random gem based on area x1 [100%]
Lost Child of Antiquity	2318	11906	Random Nourishing gem based on area x1 [8%], Twin Blood Stone Shards x1 [18%], Blood Stone Chunk x1 [1.5%]
Gravekeeper Scorpion	652	5620	Random Dirty gem based on area x1 [4%]
Strong Enemies
Merciless Watcher (Scattergun, Club & Lantern)	4563	30098	Random gem based on area x1 [25%], Blood Vial x3 [30%], Quicksilver Bullets x4 [25%], Ritual Blood(5) x1 [20%]
Merciless Watcher (Saw)	4563	30098	Random gem based on area x1 [25%], Blood Vial x3 [30%], Quicksilver Bullets x4 [25%], Ritual Blood(5) x1 [20%]
Bloodlicker	3332	20097	Random gun type gem x1 [16%]
Merciless Watcher (Mace)	4563	30098	Random gem based on area x1 [25%], Blood Vial x3 [30%], Quicksilver Bullets x4 [25%], Ritual Blood(5) x1 [20%]
Fluorescent Flower	2173	17621	Arcane Haze x3 [8%], Arcane Haze x2 [30%], Arcane Haze x1 [62%]
Labyrinth Warrior (Sword & Shield)	2753	29432	Random gem based on area x1 [25%], Blood Vial x4 [15%], Blood Vial x3 [45%], Yellow Backbone x1 [15%]
Labyrinth Warrior (Greatsword)	2753	29432	Random gem based on area x1 [25%], Blood Vial x4 [15%], Blood Vial x3 [45%], Yellow Backbone x1 [15%]
Hunter Enemies
Nameless Tomb Prospector (Kirkhammer & Repeating Pistol)	3728	28705	Random gem based on area x1 [20%]

Item Treasures	█ Notable Loot	Items Full Map Layer 1, Layer 2 & Layer 3

	Item	Location		Item	Location
Layer 1
█	Throwing Knife x3	after first ladder in side area between lamps	█	Quicksilver Bullets x2	to the right from layer lamp inside large room
█	Blood Vial x2	in treasure room that is between lamps	█	Tomb Mold (5) x1	near bridge lever in side area before layer boss
█	Ritual Blood (5) x1	end of side area before layer lamp	█	Ritual Blood (5) x2	end of side area before layer boss
█	Blood Vial x1	room after traps right from lamp	█	Thick Coldblood (5) x1	on the other side after bridge is dropped
█	Molotov Cocktail x2	large room on right path from layer lamp	█	Tomb Mold (5) x1	other side of the lowered bridge in side area before layer boss
█	Quicksilver Bullets x2	on the top of stairs in lever room	█	Molotov Cocktail x2	on the other side of dropped bridge
█	Quicksilver Bullets x3	to the right from layer lamp inside large room	█	Quicksilver Bullets x1	on the other side after bridge is dropped
█	Thick Coldblood (5) x2	to the right from layer lamp inside large room	█	Quicksilver Bullets x2	on the other side after bridge is dropped
█	Throwing Knife x2	very end of right path from layer lamp	█	Blood Vial x1	on Lower Floors with Tomb Prospector enemy hunter
█	Shining Coins x4	very end of right path from layer lamp	█	Blood Vial x2	area on Lower Floors after hunter fight
█	Shining Coins x6	large hall leading to walkway to lever room
Layer 2
█	Madman's Knowledge x1	oil covered ground in side area before layer lamp	█	Ritual Blood (5) x2	on top of circular staircase after the layer lamp
█	Kin Coldblood (12) x1	oil covered ground in side area before layer lamp	█	Quicksilver Bullets x4	treasure room after large area covered in oil
█	Blood Vial x4	Oil covered area	█	Blood Vial x4	room on Upper Floors after oil area (left)
█	Bold Hunter's Mark x2	far side of large oil covered area	█	Bone Marrow Ash x5	large area on Lower Floors near ladder to Upper Floors
█	Tomb Mold (5) x2	other side of large area in side area before layer lamp	█	Quicksilver Bullets x4	top of circular staircase from Lower Floors
█	Shining Coins x6	chest room on Upper Floors after oil area (right)	█	Quicksilver Bullets x2	up the ladder inside treasure room on Upper Floors
█	Blood Vial x3	chest room on Upper Floors after oil area (right)	█	Quicksilver Bullets x2	up the ladder inside treasure room on Upper Floors
█	Ritual Blood (5) x4	end room on Mid Floors in side area before layer lamp	█	Bold Hunter's Mark x1	on circular staircase going from Lower Floors
█	Blood Vial x3	room on Upper Floors after oil area (left)	█	Yellow Backbone x2	treasure room behind trap on Upper Floors in side area before boss
█	Blood Vial x3	room on Upper Floors after oil area (left)	█	Bold Hunter's Mark x1	small treasure room on Upper Floors
█	Blood Vial x4	circular staircase room outside of room with layer lamp	█	Ritual Blood (5) x1	on top of circular staircase on Upper Floors in side area before boss
█	Quicksilver Bullets x4	up the ladder inside treasure room on Upper Floors	█	Fire Paper x4	top of circular staircase leading to large treasure room on Upper Floors
█	Tomb Mold (5) x2	large treasure room at the end of side area before layer lamp	█	Kin Coldblood (11) x1	circular staircase room outside of room with layer lamp
█	Tomb Mold (5) x5	large treasure room at the end of side area before layer lamp	█	Odd Tempering Damp Blood Gem (5) x1	large treasure room on Upper Floors in side area before layer boss
█	Kin Coldblood (11) x1	on circular staircase leading to large treasure room	█	Ritual Blood (5) x4	large treasure room on Upper Floors in side area before layer boss
█	Blood Vial x4	on the platform in front of lever room	█	Red Jelly x1	large treasure room on Upper Floors in side area before layer boss
█	Red Jelly x1	hidden chest in the wall, in circular elevator room top floor
Layer 3
█	Kin Coldblood (12) x1	in room with Chalice Bath Messengers on Upper Floors	█	Yellow Backbone x2	room on the other side of balconies on Upper Floors in side area before layer lamp
█	Kin Coldblood (12) x1	in room with Chalice Bath Messengers on Upper Floors	█	Bold Hunter's Mark x2	platform with fountain accessed by rear tunnel or stairs from Lower Floors close to Fluorescent Flowers
█	Quicksilver Bullets x3	in top-left corner on Upper Floors ledge balconies	█	Blood Vial x4	on Lower Floors
█	Great One Coldblood x1	in top-left corner on Upper Floors ledge balconies	█	Blood Vial x4	ground level area with Tomb Prospector hunter enemy
█	Quicksilver Bullets x4	treasure room on far end of ledge balconies on Upper Floors	█	Blood Vial x4	room after hunter fight
█	Great One Coldblood x1	treasure room on far end of ledge balconies on Upper Floors	█	Great One's Wisdom x1	on pathway to the lever room

+ Layer 1 Walkthrough & Map	Click to Expand & Collapse

Click on the map for full resolution, © Bloodborne Collector’s Edition Guide by Future Press. Step 01: Be careful as you enter this room, because a pressure plate just inside the door will trigger a flaming arrow trap when stepped on, and getting hit by even a single arrow will result in massive HP loss. A large group of Labyrinth Watchers are patrolling around the middle of the room not far from a Merciless Watcher, so try to lure them back towards the entrance side of the room to eliminate the risk of getting the Merciless Watcher involved. When it comes time to fight him, rush in and light him at close range, since he has a gun and won't swap it for his club until his HP is low. When you're right next to him the shots from his gun will miss, and the swings from his lantern do negligible damage, so you can easily restore any HP you lose just by hitting him.

Step 02: A Merciless Watcher can just about be seen standing in the middle of this dark room as you enter, but don't go running straight ahead to fight him. If you hug the wall and walk around either side of the room you can get behind him to start the fight with a well placed charged R2 and follow-up of your choice to significantly weaken him. During this fight there's a strong chance that one of the Labyrinth Watchers on the floor above may drop down; keep listening for the sounds he makes to know if he's coming. Step 03: Above you on a walkway to the left, a Labyrinth Watcher will start throwing Poison Knives down at you as soon as you enter this room; if you hit him with a couple of ranged attacks of your own, you'll be able to knock him back far enough that his knives will hit the floor near his feet. Try to lure the two Hunting Dogs and Rabid Dog away from the Watcher Chieftain at the back of the room, and remember that you can shoot them to knock them over for a much easier fight. There's also a Labyrinth Watcher on either side of the room, so it's a good idea to take one of them out, and then stay on that side when fighting the other enemies, so that you don't have to worry about incoming Poison Knives. When it comes to fighting the Chieftain you can make use to the pillars to restrict his movement, or you can try to lure him near some of the explosive urns and shoot them to inflict massive damage. Step 04: To reach the lever at the bottom of the stairs in this room you'll need to get through a club-wielding Merciless Watcher, and by far the best way to start the fight is to use a Blue Elixir. After using one if you drop down from the side of the stairs you can walk around behind him and start the fight with a charged R2 from behind. If that's not an option, you should attack him with ranged attacks from the top of the stairs while he slowly makes his way towards you, and then use a well timed jumping attack once he's in range to inflict heavy damage. Step 05: A Tomb Prospector stands in wait beneath the bridge at the far end of this room, as if challenging you to approach. If you accept the challenge and want to fight him, there are plenty of things in the environment nearby that you can use to your advantage. A couple of wooden carts near the bridge provide excellent cover if you require some, and just past the bridge is a pool of oil, so if you have a fire weapon that's an excellent place to fight him. Be careful if you see him start to run away, however, because he'll often use Rope Molotovs if you give chase.

Video Guide: Bloodborne Guide - Chalice Running 101 - Great Pthumeru Ihyll by Souls Pedia

+ Layer 2 Walkthrough & Map	Click to Expand & Collapse

Click on the map for full resolution, © Bloodborne Collector’s Edition Guide by Future Press. Step 01: All of the liquid on the floor of this massive cavern is oil, so it can be very useful if you lure enemies into it and attack them with fire weapons. The only enemies here are Labyrinth Watchers, and they will blindly come running towards you as soon as they detect you, so luring them into the oil is relatively easy. Step 02: The four Ritual Blood (5) items contained within the chest at the back of this room make for tempting bait to lure you into this Nightmare Apostle nest Numerous Apostles run about on the floor, but even more are hanging down from the ceiling, so you need to be very mindful of your position, and don't go recklessly running into the room. Try to fight all of the ones on the ground without progressing past the first tree, because that's the point at which some of the ones hanging from the ceiling will start to drop down Once the first few are out of the way, systematically work your way through the room, shooting them down one at a time as you come to them. The Apostles often drop Quicksilver Bullets, so you should never run short of them, and it's even worth shooting them as they approach you so that they get knocked over and are easier to fight.

Step 03: You'll be entering this garden plaza area on the upper balcony, so use this opportunity to take out the Labyrinth Watchers and Watcher's Gravediggers that are scattered along it so that they don't become a problem later. Instead of dropping down to the main floor, go through the door on the opposite side into the elevator room. Step 04: Taking the elevator down straight away in this room would put you in an extremely bad situation, because along with two Lost Children of Antiquity there are two Labyrinth Rite Keepers and you don't have a lot of room to evade their fireballs. A much safer option is to go around the upper part of the room until you see the Keepers down below, and then use ranged attacks to kill them from there. The floor of the walkway you're on will shield you from their attacks, so you can just keep using your Throwing Knives or Molotovs until they're dead The two Lost Children are pretending to be statues, so once you're on the lower floor, look for them amongst the pots and shoot them to awaken them one at a time. Step 05: As you approach the central area of the garden plaza you'll see a Watcher Chieftain and a Wandering Nightmare patrolling around near the fountain. If you want to kill the Wandering Nightmare, you'll need to lure the Chieftain back towards you. Throwing Knives are best to accomplish that, since they don't produce any sound. Once the Chieftain is down, run up and take out the Wandering Nightmare while staying clear of the top of the stairs. A Labyrinth Warrior with a crossbow is standing with his back to you at the top of the stairs, so if you haven't disturbed him yet you can sneak up behind him and use a charged R2 into Visceral Attack combo to kill him. Killing the Warrior makes enough noise to get the attention of the Gravedigger and two Rabid Dogs that are further down the.stairs, so it's worth retreating back and using the fountain for cover straight away If the dogs follow you, use the fountain to block the Grave- digger's shots while you kill ihem.The Gravedigger will eventually lose interest in you and turn around again if you stay hidden behind the fountain, so when you see him do that, walk up and use a charged R2 from behind for an easy victory. There are some more Labyrinth Watchers and Watcher's Gravediggers around the outskirts of the room, but they're spread out enough that they're easy to pick off one at a time.

Video Guide: Bloodborne Guide - Chalice Running 101 - Great Pthumeru Ihyll by Souls Pedia

+ Layer 3 Walkthrough & Map	Click to Expand & Collapse

Click on the map for full resolution, © Bloodborne Collector’s Edition Guide by Future Press. Step 01: Hidden in a dark corner of this room you'll find a Bath Messenger that sells some handy consumables, along with the Sinister Pthumeru Ihyll Root Chalice for 16000 Blood Echoes. Before making any purchases, however, you should take out the Watcher's Gravedigger that patrols along the opposite side of the room, so that he doesn't attack you while you're browsing in the shop. Step 02: There's a Labyrinth Watcher just to the left as you enter this room, but ignore it for now and run straight ahead to where the Watcher's Gravedigger is and take him out first, since he poses a bigger threat. Once both enemies have been killed, use either of the staircases to get down to the lower part of the room and use Pebbles or Throwing Knives to lure the remaining Labyrinth Watchers away from the Merciless Watcher one at a time. You can fight them near the bottom of the sitars, and even run along the ledge without alerting the Merciless Watcher to your presence. After killing them all, sneak along one of the side walls so that you can get around behind the Merciless Watcher undetected and use a charged R2.

Step 03: It's best to enter this room through the tunnel that you can access from the bottom part of the room with the Bath Messenger, because it allows you to approach the enemies from an advantageous position. When you exit the tunnel you'll see a Watcher's Gravedigger with a gun standing near the stairs straight ahead; after he spots you, hide behind one of the support columns of the nearby archway so that he starts walking towards you, and then quickly run over and kill him That should ensure none of the other enemies become aware of your presence, which will make dealing with them much easier. A Wandering Nightmare can usually be found somewhere around the central fountain, so try to locate it, and if it's not too near one of the other enemies, kill it quickly. Next you'll need to kill the Gravedigger standing with his back to you, so sneak up behind him and use a charged R2 for an easy victory. Continue around the area clockwise, taking out the next Gravedigger, followed by the Watcher Chieftain (both of whom also have their backs to you). If you look over the edge of the area you should see two Rabid Dogs standing below, and it's worth using some ranged attacks to either kill them outright, or lure them up the stairs towards you. The remaining threats are the Fluorescent Flowers that are in each of the corners of the lower part of the room. While you're still on the upper section, it's worth using some ranged attacks on the ones you can reach, as it's much safer taking them out from up there. When you finally have to go down to finish them off, work your way around the room taking them out as you come to them while using the pillars for cover. If you get close enough, it should only take a couple of hits to the flower to finish one off. Step 04: Try to lure the Gravekeeper Scorpion near the entrance of this room back towards you, so that you can fight it away from the Tomb Prospector that's standing near the opposite wall. If he hasn't detected you, sneak around the right-hand side of the room so that you can get behind him and start with fight with a charged R2 from behind There's a pool of oil in the tunnel you use to reach this room, so if you have a strong fire weapon you may want to lure him back into it. The other end of the room has two more Gravedigger Scorpions as well as two Labyrinth Warriors, so make sure you use Pebbles to lure them away one at a time to keep the fights manageable. The two Labyrinth Warriors both have Fire element weapons, so don't enter the pool of oil near them during the fight. Step 05: Along with a gun toting Merciless Watcher, there's also two Lost Children of Antiquity guarding the lever in this room; one pretending to be a statue near the front of the upper part, and another clinging to the ceiling. Use ranged attacks from the stairs to knock them down one at a time so that you can fight them away from the Merciless Watcher. Once they've been defeated, sneak around the upper part of the room so that you can get behind the Merciless Watcher and use a charged R2 to start the fight.

Video Guide: Bloodborne Guide - Chalice Running 101 - Great Pthumeru Ihyll by Souls Pedia