This is the most challenging and time consuming trophy of the game. The Pthumerian Queen is a special boss at the end of one of the chalice dungeons however there is a long sequence of events that must be completed before you are able to face her, including some extremely challenging boss fights. I would suggest doing this after beating  Mergo's Wet Nurse but before facing the final storyline boss. Chalice Dungeons also persist in NG+ however, so it won't matter if you decide to complete this as the last step before your platinum.

There are two varieties of Chalice Dungeons:

• Root Chalice Dungeons: These are completely randomly generated, and offer a different experience every time you create one
• Standard Chalice Dungeons: These are fixed, with set enemy, chest and boss placements. The contents of the chests may vary however every time you create one.

For this step we will only be completing standard chalice dungeons, meaning it is not required to complete any of the completely randomly generated dungeons. Unfortunately you may be required to recreate previous dungeons multiple times in order to farm the ingredients for later dungeons.

The first step is to complete the Pthumeru Chalice Dungeon. The chalice to create this dungeon drops from  Blood-starved Beast and requires the following ingrediants:

• Ritual Blood (1) x 1 : Can be purchased from the Bath Messengers in the Hunters Dream
• Blood Echoes x 1000

The boss on the third and final layer of this dungeon will drop the Central Pthumeru Chalice

You now must complete the Central Pthumeru Chalice Duengeon, which requires the following ingredients:

• Ritual Blood (2) x 6 : Can be found in the previously completed Pthumeru Chalice Dungeon, as well as other depth 1 and depth 2 dungeons. You may need to create multiple Pthumeru Chalice Dungeon runs in order to farm the 6 required. It also eventually becomes purchasable from the Bath Messengers in the Hunters Dream
• Blood Echoes x 1800

The boss on the third and final layer of this dungeon will drop the Lower Pthumeru Chalice.

You now must complete the Lower Pthumeru Chalice Dungeon, which requires the following ingredients:

• Ritual Blood (3) x 9 : Can be found in the previously completed Central Pthumeru Chalice Dungeon, as well as other depth 2 and depth 3 dungeons. You may need to create multiple Central Pthumeru Chalice Dungeon runs in order to farm the 9 required.
• Blood Echoes x 3200

The boss on the fourth and final layer of this dungeon drops the Defiled Chalice. Also be sure to pick up the Workshop Haze Extractor from the area behind the door just before the second layer boss.

You now must create and partially complete the Ailing Loran Chalice Dungeon. The chalice to create this dungeon drops from the optional boss  Amygdala and requires the following ingredients:

• Ritual Blood (4) x 9 : Can be found in the previously completed Lower Pthumeru Chalice Dungeon, as well as other depth 3 and depth 4 dungeons. You may need to complete multiple Lower Chalice Dungeon runs in order to farm the 9 required.
• Coldblood Flowerbud x 4 : Can be purchased from the insight Insight Bath Messengers. 3 can also be found in the Nightmare Frontier
• Blood Echoes x 5500

You only need to complete enough of this chalice dungeon to farm 2 x Bastard of Loran, which you may obtain as early as the first layer. You should however proceed as far as the second layer where you will find the Beast Claw Weapon which is required for Hunter's Essence. You may also unlock  Blood Gem Master from drops within this dungeon or any others of the same depth or higher.

You now must complete the Defiled Chalice Dungeon, which requires the following ingredients:

• Ritual Blood (4) x 9 : Can be found in the previously completed Lower Pthumeru Chalice Dungeon, as well as other depth 3 and depth 4 dungeons. You may need to complete multiple Lower Chalice Dungeon runs in order to farm the 9 required.
• Bastard of Loran x 2 : Previously obtained from the Ailing Loran Chalice Dungeon
• Arcane Haze x 22 : By this point you should have found the Workshop Haze Extractor within the Lower Pthumeru Chalice Dungeon, which you can use this to extract Haze from Chalice Dungeon Materials at the Workshop. I would advise to purchase Ritual Blood (1) from the Bath Messengers and extract Haze from that to prevent you from inadvertently destroying materials you may want for later Chalice Dungeons.
• Blood Echoes x 5500

It is at this point where the chalice dungeons become more challenging that the storyline game. This is significantly even more so given that in the Defiled Chalice Dungeon you only have half your health, meaning it's entirely possible to be one shot by bosses and even mobs. This will likely be the make or break point for you for this trophy. The boss on the third and final layer drops the Great Pthumeru Ihyll Chalice, which is the chalice that the trophy boss can be found on.

Before you can undertake the Great Pthumeru Ihyll Chalice you need several additional ingredients, one of which is 4 x Red Jelly. 2 of these can be found in a chest in the lower floor of the lecture building, whilst the other 2 you need to farm from Chalice Dungeons.

The chalice dungeon I suggest for this is the Great Isz Chalice Dungeon. The chalice to create this dungeon drops from  Ebrietas, Daughter of the Cosmos and requires the following ingredients:

• Ritual Blood (5) x 9 : Can be found in depth 4 and depth 5 dungeons. Hopefully you now have a large enough stockpile that you shouldn't have to farm them.
• Pearl Slug x 3 : Can be purchased from the Insight Bath Messengers
• Arcane Haze x 25 : As per the same method as described in the defiled chalice dungeon
• Blood Echoes x 11500

While the two Red Jellies are on layer 3, the bosses on both layer 1 & layer 2 are very simple, especially compared to those of the defiled chalice that you previously completed. Once you get to the third layer you will eventually come to a corridor with a boulder continually rolling down it. When the boulder rolls past, follow it down the corridor and look for a side room to the left. There are chests in here, two of which contain Red Jelly's.

You can now complete the Great Pthumeru Ihyll Chalice Dungeon, which requires the following ingredients:

• Ritual Blood (5) x 9 : Can be found in Depth 4 and Depth 5 dungeons. Hopefully you now have a large enough stockpile that you shouldn't have to farm them.
• Red Jelly x 4 : You should have these from the previous step
• Arcane Haze x 25 : As per the same method as described in the defiled chalice dungeon
• Living String : This is obtained from the Mother Brain in the Nightmare of Mensis. Refer to Weapon Master for additional instructions
• Blood Echoes x 11500

 Yharnam, Pthumerian Queen is the boss of the third and final layer and the trophy will unlock immediately after defeating her.

The biggest challenge are the bosses, especially those of the Defiled Chalice and Great Pthumeru Ihyll Chalice. Hopefully it will be a smoother experience for you than it was for me, however I will share the strategies that I used:

• Keeper of the Old Lords (Layer 1, Defiled Chalice) : Keep her locked on and at a safe distance. Learn her timings and get in close using jumping attacks (forward + ) with a heavy weapon to get a few hits in before retreating back to a safe distance. Alternatively if you have 15 arcane the Beast Roar Tool will knock her over. Using this along with a fast weapon means you can stunlock her.
• Watch Dog of the Old Lords (Layer 2, Defiled Chalice) : A safe strategy for this fight is to lock on to the target and continue to walk backwards along the circle near the middle of the arena, doing continuous slow, circular laps. While doing this none of his regular attacks will be able hit you. When he does his head swipe walk in and hit him once on the nose before dashing back and continuing to walk backwards around the arena. Your only threat is his charge that he occasionally does after a head swipe; if his body engulfs in flames them immediately start dodging because he is about to charge and it will one shot you if it hits you.
• Amygdala (Layer 3, Defiled Chalice) : The same strategy described in  Amygdala; don't lock on, get in behind to her tail where it's safe and make sure you rotate with her. Every time she jumps to reposition herself make sure that you roll under her legs back to the safe zone. This is a long fight since she is heavily armoured around her tail and back legs.
• Pthumerian Descendant (Layer 1, Great Pthumeru Ihyll Chalice) : Similar to the tactic for the Keeper of the Old Lords, keep him locked on and learn the timings for a jumping attack (forward + ) with a heavy weapon to get a few hits in. He becomes much more aggressive in his second phase. Don't be afraid to disengage your lock and run back to the other end of the arena when things get overwhelming in order to heal up and get some distance between you and him. If you do retreat I would suggest weaving around the columns to make his attacks less likely to hit you while you've got your back to him.
• Bloodletting Beast (Layer 2, Great Pthumeru Ihyll Chalice) : During the first phase keep him locked on and stay just out of range of his attacks. There are two moves you want to look out for. The first is his two frontal swipes - You can dash in after the first swipe to get a couple of strikes on his legs before dashing away. The second attack is when he leans over and spits poison at you - immediately dash up to be right in front of his body where his poison will miss you altogether, leaving you free to hit him in his stump where his neck is - this is his weak spot and the second time he does this should be enough to stun him and set him up for a visceral attack. Other than that dash away when he corners you to put distance between yourself and him again. During the second phase when he gains his serpent like head you need to change your strategy since he has some devastating mid and long range attacks that will quickly tear you apart if you stay at range. Instead of keeping your distance you need to be aggressive and get right under him - he doesn't do his ground stomps as frequently during this phase so it's not as deadly as the first phase. Get a few hits in and quickly back out to recover your stamina and heal up before getting back into the fray. Stay aggressive and don't let him take control with his mid and long range attacks.
• Yharnam, Pthumerian Queen (Layer 3, Great Pthumeru Ihyll Chalice) : This is probably the easiest fight of this chalice. Her attacks are easy to dodge and it's easy to get in close to do damage. Be very cautious however, immediately stop hitting her as soon as you hear a baby start crying otherwise you become stunned and she has the potential to kill you before you recover. I was sticking with single jump attacks (forward + ) before retreating to avoid her stuns altogether. If you do get stunned you can mash  and  to try and break out early. The other mechanic to watch out for is when she dissolves into the ground and creates two clones. Get some distance and identify which is the real Queen; she can be identified since she is the one firing 3 projectiles instead of one. Once you have identified her concentrate on taking out the clones first since they go down in a single hit.

Here's to wishing you the best of luck for this very well-earned gold trophy.

Missable Note: If you start NG+ your dungeons will carry over and they will not incease in difficulty like the rest of the game. However, if you didn't kill the Brain of Mensis in the Nightmare of Mensis area you'll miss the Living String which is required for the last chalice ritual. This means you'd have to play through most of the game again until you reach Margo's Loft (Middle) to obtain the string from there.
 

The long way down - a brief Pthumeru Chalice Walkthrough

In order to reach the Pthumerian Queen you need to make your way through numerous dungeons. In the following mini-walkthrough you will find all the crucial information: Which Chalice Dungeons you need to play, how to obtain the chalices and where to find the ritual materials.

1. Pthumeru Chalice Dungeon

Chalice location:	Dropped by the Blood-starved Beast.
Ritual Materials:	Ritual Blood (1) x2 (found in story mode, can also be bought at the Messenger's Shop)
Blood Echos:	1,000

2. Central Pthumeru Chalice Dungeon

Chalice location:	Dropped by the Watchdog of the old Lords, the boss of layer 3 of the Pthumeru Chalice Dungeon.
Ritual Materials:	Ritual Blood (2) x6 (can be found inside depth 1 and depth 2 Chalice Dungeons)
Blood Echos:	1,800

 

3. Lower Pthumeru Chalice Dungeon

Chalice location:	Dropped by the Pthumerian Descendant, the boss of layer 3 of the Central Pthumeru Chalice Dungeon.
Ritual Materials:	Ritual Blood (3) x9 (can be found inside depth 2 and depth 3 Chalice Dungeons)
Blood Echos:	3,200

4. Ailing Loran Chalice Dungeon

Chalice location:	Dropped by the optional boss Amygdala, you need this dungeon for Ritual Material, but you will also find one of the weapons needed for Hunter's Essence here.
Ritual Materials:	Ritual Blood (4) x9 (can be found inside depth 3 and depth 4 Chalice Dungeons) Coldblood Flowerbud x4 (Found in Nightmare Frontier, can be bought at Messenger's Shop after defeating Amygdala)
Blood Echos:	5,500

 

5. Defiled Chalice Dungeon (you'll start cursed and only have 1/2 health)

Chalice location:	Dropped by the Bloodletting Beast, the boss of layer 4 of the Lower Pthumeru Chalice Dungeon.
Ritual Materials:	Ritual Blood (4) x9 (can be found inside depth 3 and depth 4 Chalice Dungeons) Bastard of Loran x2 (Found in the Ailing Loran Chalice Dungeon) Arcane Haze x22 (dropped by the Fluorescent Flower Enemies in Chalice Dungeons or Byrgenwerth, can also be won by using the Arcane Haze Extractor found in the Ailing Loran Chalice Dungeon)
Blood Echos:	5,500

6. Great Pthumeru Ihyll Chalice Dungeon

Chalice location:	Dropped by Amygdala, the boss of layer 3 of the Defiled Chalice.
Ritual Materials:	Ritual Blood (5) x9 (can be found inside depth 4 Chalice Dungeons, 3 can be found in the Upper Cathedral Ward, in front of the gate where you need to use the Orphanage Key) Red Jelly x4 (2 red jelly is found on the first floor of the Lecture theatre, while another easy two can be found on the 3rd floor of the Great Isz Chalice Dungeon (Chalice dropped by Ebrietas) in the treasure room) Arcane Haze x25 (dropped by the Fluorescent Flower Enemies in Chalice Dungeons or Byrgenwerth, can also be won by using the Arcane Haze Extractor found in the Ailing Loran Chalice Dungeon) Living String x1 (dropped by Brain of Mensis in the Nightmare of Mensis: drop it into the abyss like shown in the video for  Weapon Master, then follow it down using a cage elevator and kill it for the string - you might want to do the Make Contact gesture in front of it for about one minute first for a rare Rune though)
Blood Echos:	11,500

How to find the boss

You will find the Pthumerian Queen at the end of the third layer of the Great Pthumeru Ihyll Chalice Dungeon. Congratz on making it that far!
 

Preparation

Attire with high Blood defense is invaluable here. A full stock of Health vials, a +10 upgraded weapon, and above level 120, and you should be golden.
 

Boss Fight Strategy

Yharnam is relatively easy compared to the previous bosses in the chalice dungeons. When the fight starts, you'll see her standing there. She will primarily use blood attacks, and has a few AOE attacks too, so watch out. She will fire blood bolts at you from a distance, and when you get close, she will try and swing. Don't let her, instead use a fast weapon like the Saw Cleaver to stun lock her. Hit her about 3 times, and you will notice that she is no longer getting stun locked. She will now be charging her AOE, so roll away, and then repeat as necessary. Eventually she will spawn 2 other clones, that will die after 1 hit. Simply sprint around the arena attacking each clone to prevent any shenanigans with them later on. Yharnam's skills will have upgraded, with faster blood shooting, a bigger area of effect attack, and a blood dagger that can deal more damage. The tactics don't necessarily change though, just keep sprinting up to her, deal out 3 hits, then roll away to safety. Yharnam will upgrade a final time, getting a blood coated sword in the process. Tactics still don't change, but be wary of any damage you take, as you will take a lot at this stage. When you kill her you will feel a rush of euphoria, all the effort of playing through these dungeons has finally payed off, and you've earned a nice gold trophy to boot!
 

Yharnam, Pthurmerian Queen Video Guide

Video by PowerPyx.