The Oathbound side quest in Valor Mortis requires players to locate and defeat seven distinct Eternal Guardsmen scattered across various game chapters. Successfully vanquishing these foes yields their Emblems and ultimately grants access to a hidden area, alongside the 'We Wear The Same Colours' achievement.
This guide details the locations of all seven Eternal Guardsmen and their corresponding Emblems, presented in the order they appear from the Battlefield to the Fortress.
The Battlefield: Valen Duroc's Emblem
Begin at the Onslaught Arch Lantern in the Battlefield. Proceed towards the bridge behind you and follow the cliff edge. Turn right, cross the subsequent bridge, and continue forward until you encounter a sleeping soldier beside a crate. Approach him, then turn left to find a Nephtoglobin node. Shoot this node and advance to confront Valen Duroc. Defeating him rewards you with a Mysterious Stone Pyramid and Valen Duroc's Emblem.
Valley of Ashes: Lucien Lafontaine's Emblem
From The Climb Lantern in the Valley of Ashes, ascend using the elevator. In the mid-section, crouch through the opening on the right to enter a room containing mutant enemies. After some climbing, you will find a ladder; ascend it. Upon exiting, proceed right along the wooden platforms. Ignore the next ladder on your left and continue along the wooden platforms past it. At the end of these platforms, you will be beneath a bridge with enemies. Clear them out, traverse under the bridge, and begin ascending the corrupted path. Before stepping onto the wagon, look down to your right to discover an easily missed path leading to Lucien Lafontaine. Defeat him to obtain a Mysterious Stone Pyramid and Lucien Lafontaine's Emblem. 
Fallen Village: Etienne Vauclair's Emblem
Starting from the Marketplace Lantern in the Fallen Village, proceed through the wagon behind you. Continue straight, then turn left before the wooden gate. You will reach Nephthtoglobin formations and platforms suitable for wall-riding. Utilize these to reach the hut on the opposite side of the cavern, which is followed by further parkour challenges. After the platforming section, enter a cave containing a Nephthtoglobin node. Descend within the cave to locate and shoot the node. Exit the cave and enter the passage above, now unblocked, to find Etienne Vauclair. Defeating him yields a Mysterious Stone Pyramid and Etienne Vauclair's Emblem. 
Caverns: Jean Pierre Polnaire's Emblem
From the Old Mine Lantern in the Caverns, proceed straight into the mines. Utilize the crate in the room, which can be frozen with Stasis, to reach the opposite side. Ascend the ladder there and cross the room again. To do this, interact with the crate's lever, freeze it midway, and use it to cross. Climb the ladder, then unfreeze the crate and quickly use it to return to your original side. A ladder can be kicked down here for easier access later. In the room ahead, you will encounter Jean Pierre Polnaire. Defeating him will grant Jean Pierre Polnaire's Emblem and a Mysterious Stone Pyramid. 
Cursed Hollow: Severin Susse's Emblem
From the Deep Water Lantern in Cursed Hollow, enter the house with blue banners on your left, a shortcut unlocked through exploration. After exiting this house, proceed straight and then left into a poisonous area guarded by Volunteer enemies. Continue past them into the swamp ahead, which contains poisonous water, hay bales, and enemies. Navigate through this swamp towards the University in the distance, but do not enter through the gates. Instead, turn right to find more huts. One hut on the left side, as you approach, will have the Eternal Guardsmen's symbol painted on it; Severin Susse is located just outside this house. Defeat him to receive a Mysterious Stone Pyramid and Severin Susse's Emblem.
City of Orphans: Adrien Casell's Emblem
From the Shamble Street Lantern in the City of Orphans, proceed through the gate on your left (this shortcut requires prior unlocking). Descend the stairs and head straight onto the main street, then turn left under the bridge. Continue straight, past a Volunteer enemy, and enter a poisonous area (requiring the Shield Transmutation from the University for protection). The Eternal Guardsmen's symbol is painted on the wall of the house to your left. Follow the alleyway on your left while traversing the poison. Upon reaching a mutant enemy, turn left again. At the end of this back alley, climbable crates lead into a house. Jump from the broken wall inside this house onto the street, then turn left to encounter Adrien Casell. Defeating him yields a Mysterious Stone Pyramid and Adrien Casell's Emblem. 
Fortress: Bastien Beaufort's Emblem
From the Courtyard Lantern in the Fortress, go through the archway ahead adorned with a tricolore. Turn left and defeat the Eternal Guardsman present. This foe is not Bastien Beaufort. Instead, after defeating him, loot the corpse where he stood to acquire Bastien Beaufort's Emblem and a Mysterious Stone Pyramid. 
Completing The Oathbound Side Quest
Upon collecting all seven Emblems and the Mysterious Stone Slab, return from Bastien Beaufort's location in the Fortress and proceed straight to the opposite end of the hallway. You will find a stone-blocked doorway marked with the Eternal Guardsmen's symbol. Interact with this symbol after acquiring all seven Emblems to enter a grand dining hall. Here, you will find The Codex of the Oathbound on a table, completing the quest. The room also contains four chests offering one Quintessential Blade Material, one Empty Ferrofluid Flask, and six Remnant's Return consumables. 
