A Plague Tale Requiem has 16 Knife Locations on Narrative difficulty. The Knives are necessary to open the 10 Secret Chests. You need 1 Knife per Secret Chest, so 10 total. Because there are 16 Knives on Narrative difficulty you can miss a few and still get all Secret Chests. But on higher difficulties there are fewer Knives, some spawns will be removed with each higher difficulty level.
The knives don’t give a trophy or achievement directly, but because they are needed for the Secret Chests they are technically also a Collectible type needed for trophies & achievements.
IMPORTANT: You need Knives to open the Secret Chests. Don’t break knives on enemies! They are one-use only and break when doing a single kill. Always keep 1 Knife in your inventory to open the next Secret Chest. Sometimes you automatically use a knife against melee enemies to prevent a killing blow, when that happens restart the checkpoint immediately. If you reach a Secret Chest but don’t have a knife, you must replay from the last Knife Location onward via Chapter Select. More knives spawn on Narrative Difficulty (Easy) and fewer spawn on higher difficulties. For this reason it’s highly recommended to play on the lowest difficulty to get all possible knives. This guide includes all possible spawns from Narrative difficulty. If you play on Normal or Hard (or New Game+) some Knives will not be there but the remaining ones are in the same locations. Basically, each higher difficulty level removes a few knives from the game but the ones that are still there have the same location as in lower difficulties.
» For All-in-One Collectible Guide including all other types see: A Plague Tale Requiem All-in-One Collectible Guide.
Chapter 1: Under a New Sun
NO KNIVES IN THIS CHAPTER!
Chapter 2: Newcomers
1 Knife is automatically picked up in this Chapter (story-related, unmissable).
Chapter 3: A Burden of Blood
1 Knife is automatically picked up in this Chapter (story-related, unmissable, in the Warehouse at the end when picking up Sling).
Chapter 4: Protector’s Duty (1 Hugo’s Herbarium, 2 Souvenirs)
Knife #1
Sub-Chapter: Risky itinerary
Objective: Reach the order’s boat
At the start of the first real enemy encounter in this chapter, you can find a knife sticking out a wooden fence in the middle of the area, in the outside area before engaging the enemies. Don’t use the knife, you need it to open the Secret Chest collectible. Keep in mind that this, like many other knives, only spawn on NARRATIVE DIFFICULTY! The knife won’t be there on higher difficulties or in New Game+. So if a knife doesn’t spawn for you, change the difficulty via the Gameplay Settings and replay the last Sub-Chapter to make it spawn.
Knife #2
Sub-Chapter: The mass grave of the marshes
Objective: Get to Joseph at the pontoon
During a big enemy encounter with a large burning bonfire in the middle of the area. About 20 meters before the bonfire is a tree stump with a knife sticking out of it.
Chapter 5: In Our Wake
Knife #3
Sub-chapter: On the River
Objective: Cross the construction site
While crossing the rat-infested area, after reuniting with Lucas, don’t continue just yet towards the braziers (where the story wants you to use fire sticks to advance). Instead, if you look behind and above you, you should see a ladder behind held by a chain. Destroy the chain and then keep climbing up. At the top, you can find the knife under a roof.
Knife #4
Sub-Chapter: Grapples
Objective: Get rid of the bandits
After finishing the boat ride, when you first get to carry the crossbow, a knife is straight ahead of you on a fallen tree. During an enemy encounter when an armored soldier attacks you and you get a tutorial prompt for using the crossbow.
Chapter 6: Leaving all Behind
Knife #5
Sub-Chapter: Connection
Objective: Find a way out
After the first group of enemies in the quarry, after you go through a door to escape them there’s a knife sticking out of an old mine cart (2nd enemy group in quarry).
Knife #6
Sub-Chapter: The orches
Objective: Reach the building
Immediately after leaving the quarry, there’s another enemy encounter, go straight ahead and climb up the white ledge to find a supply crate, with a knife next to it.
Knife #7
Sub-Chapter: Hunting Lesson
Objective: Exit the dye works
After learning how to control rats, in the next enemy encounter in an outside area, a knife can be found on the right side, at a wooden cart where some rats are. You can hold 
, aim at the rats and press 
/ 
on them to steer them to an enemy, then the area is clear for you to pick up the knife.
Chapter 7: Felons
Knife #8
Sub-Chapter: Arrival at the beach
Objective: Cross the beach
Shortly after the beginning of the chapter you will reach a beach. After walking a few steps, Hugo will charge into some seagulls, right after that you will see a little house on the left. Enter it, a knife is on a mattress. Then through the left window. At the end of that path after the window you will find the feather on the ground.
Knife #9
Sub-Chapter: The combat according to Arnaud
Objective: Reach Sophia’s boat
When you go outside and reach an enemy encounter at a beach, you get the ability to command Arnaud to attack enemies. Here at this beach is a knife, on the right side by a shipwreck, check the rocks before the shipwreck.
Chapter 8: A Sea of Promises
NO KNIVES IN THIS CHAPTER!
Chapter 9: Tales and Revelations
Knife #10
Sub-Chapter: Exploration
Objective: Follow the bird statues
In the huge exploration area, head towards the mountain with the two big peaks. Just before heading up the path to the mountain, on the left is a ruined tower. Shoot through its window to hit the lock on the door. Then you can enter the door, directly behind it is a knife (if you already have a knife, then pick it up after opening the Secret Chest). Climb up the ladder to find a Secret Chest and break it open with the knife.
Chapter 10: Bloodline
Knife #11
Sub-Chapter: Approaching the fort
Objective: Reach the fort
After Hugo runs across a field, you will soon reach an enemy encounter. During this enemy encounter, follow the paved path in the middle until the 2nd ruined building on the right side. In that building is a knife (ground floor) and a Tool Chest (upper floor).
Knife #12
Sub-Chapter: The chapel
Objective: Survive the Assault
After the chapel (crypt) there will be a big enemy encounter where you fight waves of enemies. During this encounter, in the right corner is a knife (don’t break it so you have it for the Secret Chest in next chapter).
Chapter 11: The Cradle of Centuries
Knife #13
Sub-Chapter: Byzantine technology
Objective: Explore the underground area
In a room with a workbench (reached as part of the story, unmissable), crouch through the hole in the left wall, behind it you can shoot Ignifer slingshot at explosive barrels to blow open the gate. Go through the gate, in the next corridor turn right to find a skeleton with a knife sticking out of it.
Chapter 12: The Life We Deserve
NO KNIVES IN THIS CHAPTER!
Chapter 13: Nothing Left
Knife #14
Sub-Chapter: Fresh Air
Objective: Reach the port
When you reach an area with human enemies, in the left-end corner of the area, sticking out of a cart that’s rolled on its side (resource chest in front of it, guard patrolling near it).
Knife #15
Sub-Chapter: Ruined village
Objective: Reach the port
After leaving the enemy area where the last Knife was, you will reach a burning village with more enemies. Take the path on the left, through the barn, then through a house (ground floor). You find the knife after passing through the house, on a table with flowers.
Knife #16
Sub-Chapter: Ruined village
Objective: Reach the port
From the previous knife, finish the area with the enemy encounter. In the next area you enter a house with a workbench. Climb up the ledge in this house, then drop down the balcony on the left. Immediately after, turn left to see the knife.
Chapter 14: Healing our Wounds
NO KNIVES IN THIS CHAPTER!
Chapter 15: Dying Sun
NO KNIVES IN THIS CHAPTER!
Chapter 16: King Hugo
NO KNIVES IN THIS CHAPTER!
Chapter 17: Legacy of De Rune Family
NO KNIVES IN THIS CHAPTER!
That’s all of the Knives in A Plague Tale Requiem. For all other Collectibles see A Plague Tale Requiem Collectibles Guide.
There is another knife in Chapter 2 right after the flower you pick up for Hugo. You head down some stairs and you can follow some up and around to a bridge. There is a chest here and before you turn left for chest if you go straight there is a knife.
Chafter 4: in maximum difficulty you could find one. When at knife #1 there is a hole at your left, go inside and just look behind you.
Very useful! Thank you!
And yes, I confirm what Guigs says: in Chapter 4 and hard difficulty, when at the ground level where 2 soldiers guard the large gate and before reaching the section with the 2nd chest,
the hard-level knife is right behind you after going through the passage between planks, .
Too bad that this game has an interesting story and allegedly beautiful graphics (for those who can buy an over 1k euro graphic card (the starting price of a 3700 in Europe), but garbage gameplay, especially starting with chapter 3. The only thing that they got right was controlling the rats with W, A, S, and D cos it in Innocence it was only with the mouse struggling with a very bad camera view.