Scattered across Valor Mortis are 7 Eternal Guards, elite soldiers who each carry a Mysterious Stone Pyramid. Defeat all 7 and the pyramids combine into the Mysterious Stone Slab, which opens a secret door in the Fortress region.
Hunting down the Eternal Guards is part of “The Oathbound” side quest. Defeat all 7, then read the letter waiting in the secret room to complete it, which unlocks the “We Wear the Same Colours” Trophy/Achievement.
Eternal Guard 1
The first Eternal Guard can be found near the “Onslaught Arch” Lantern in The Battlefield region.
Backtrack from the Lantern until you get to the crossroads near the start of the game, here take the left path and shoot the red orb to destroy the vines blocking the path.
Behind the vines you will find the 1st of the Eternal Guards that you need to defeat.
Defeat him to get Valen Duroc's Emblem and a Mysterious Stone Pyramid.
Eternal Guard 2
The second Eternal Guard can be found near the “Demolished Path” Lantern in the Valley of Ashes region.
Just after you swing under the bridge on your way from "The Climb" Lantern heading towards the shortcut back to the "Demolished Path" Lantern, take the path going down to the right.
At the bottom of the ramp, go left to find the 2nd Eternal Guard.
Defeat him to get Lucien Lafontaine's Emblem and a Mysterious Stone Pyramid.
Eternal Guard 3
The third Eternal Guard can be found near the “Fields” Lantern in the Fallen Village region.
Advance through the level until you reach the wooden gate just before the carriage where you unlocked the Blunderbuss Weapon, turn to the right immediately after going through the gate (before the carriage).
Here you will need to use the Nephtoglobin Formations ability (unlocked in the main story in Vale of Wrath) to pull out the platforms to reach the wallrunning section.
Continue on this path and you will soon find the Eternal Guard in a cave.
Defeat him to get Étienne Vauclair's Emblem and a Mysterious Stone Pyramid.
Eternal Guard 4
The fourth Eternal Guard can be found near the “Old Mine” Lantern in the Caverns region.
From the Lantern, go down the main story path and climb the stairs to pull the lever at the top, this will raise the box in the middle of the room, while the box is raised half way use the Stasis Transmutation to freeze the cogs so that the box stays in place.
Then jump on the box, release the Stasis so that the box continues to raise, then turn around 180º and jump across to the highest platform behind you where you will find the Eternal Guard.
Defeat him to get Jean Pierre Polnaire's Emblem and a Mysterious Stone Pyramid.
Eternal Guard 5
The fifth Eternal Guard can be found near the “Village Church” Lantern in the Cursed Hollow region.
From the Village Church Lantern, go through the shortcut you unlock from the other side near the end of the level and continue heading straight across the swamp in front of you.
Head slightly to the left as you advance to go around the hay and you will eventually see the Eternal Guard on your right next to a house.
Defeat him to get Severin Susse's Emblem and a Mysterious Stone Pyramid.
Eternal Guard 6
The sixth Eternal Guard can be found near the “Shamble Street” Lantern in the City of Orphans region.
Go down the stairs and head left, when you reach the area with the ramp on your left with 2 tall enemies, instead of going up the ramp go straight ahead and use the Shield Transmutation to go through the poison mist. Follow this path through the house to reach the Eternal Guard.
Defeat him to get Adrien Casell's Emblem and a Mysterious Stone Pyramid.
Right next to where he stands you'll also find “A Letter from Bastien Beaufort”, a letter that hints at what the Eternal Guards are hiding.
Eternal Guard 7
The seventh Eternal Guard can be found near the “Courtyard” Lantern in the Fortress region.
From the Courtyard, go through the gateway on the left which leads to a big hall, at the left end of the hall you will see an enemy standing over the corpse of the Eternal Guard.
Defeat the enemy then loot the final Mysterious Stone Pyramid from the corpse on the ground near the end of the hall.
Opening the Secret Door
Once you've defeated all 7 Eternal Guards, the pyramids combine into the Mysterious Stone Slab. Take it to the secret door near the “Courtyard” Lantern in the Fortress region.
In the same hall as the 7th Eternal Guard, but this time go to the right hand side and interact with the Symbol at the end and the wall will slide open to reveal a hidden room.
The Codex of the Oathbound
Inside the secret room, read “The Codex of the Oathbound” to complete “The Oathbound”. This rewards you with 10,000 Catalysts and unlocks the “We Wear the Same Colours” Trophy/Achievement.
