Doom Eternal: The Ancient Gods Part One, like the main game, has collectible Codex entries. There are 13 pages to locate across the expansion’s three missions, and finding them all will net you an achievement or trophy. If you need some help to locate each one, or a certain page that you are missing, allow us to give you a hand.
UAC Atlantica Facility
The first entry is found in the very first area. Defeat the demons that spawn on your way to your first objective marker. Head inside the marked room and grab the blue keycard. Defeat the next enemy wave and head up to the door on the top platform on the far left of the facility.
Head inside, walk past the locked Red Access door to your left, and go down the stairs. The Codex entry will be in front of you.
The next page is in the area before you push a lever that drops a shipping container that carries you to a new region. After defeating the demons here, head up the stairs to a small platform with a medkit and some shotgun ammo. The page is here.
Following your first Archvile encounter, you will need to activate a fan to blow you across a large stretch of ocean to reach the other side. Do this, and jump to the swing bar to be launched across the way.
Once you land, defeat the demons that spawn. Before you use the nearby bars to swing to your next objective marker, head down the lowest path until you reach a dead end. Look for another path next to this locked door. Jump to it and follow it around to find the third Codex entry.
The final entry in this mission can be found after your first Doom Hunter encounter. Head through the door, and before you walk up the stairs, follow the path underneath them. The entry is hidden here.
The first Codex page in the second mission can be easily missed. On the platform in the screenshot below, double-jump and dash across to the small platform across from you.
Drop down, and defeat the Cyber Mancubus that spawns. Grab the Rad Suit and dash across the water to the ammo on the other side. Turn around and you will spot the page under the water. Dive down to grab it.
The next page is located in a misty area. You have to defeat more demons, and this can be tricky, as your field of view is restricted. After that battle, look for the area in the screenshot below. It should be on the back wall near the fallen pillar you have to climb up to reach the next area.
Look for a small hole in the ground by this wall. Dive into the water and follow it until you can re-emerge. Defeat the Hell Knight and Cyber Mancubus, and then dash across the gap to grab the entry.
The third entry can be gathered after you defeat a Pain Elemental around some climbing walls and swing bars. Platform your way up to the giant pillar that stands across from a broken walkway. Jump and dash across the gap to grab the page.
The last Codex page can be discovered in the green wolf section. You have to stay inside its safety zone or your health will deplete, so do so while you navigate this area and take out demons along the way. Once you reach an area where the air is safe to breathe, double back on yourself to the screenshot below.
You will see the Codex page on a platform above. Jump up to grab it.
To locate the first page, reach the area where you punch down a tree in the screenshot below.
Before you head across, look over the ledge next to it. Drop down to grab the page, and use the climbing walls to head back to the tree.
The next entry is found when you see the Luminarium building for the first time. It will be when the Father speaks to you about his physical form. Jump to the platforms to your right to grab the Codex.
The third page is found in a room where you have to push a skull button to open the doors to the Luminarium. You can’t miss it, as it is to your left.
The fourth entry can be found after defeating a wave of demons in a left or right room, depending on which path you take. Head into the next room where the floor is missing.
Punch the lever to activate a gravlift. Double-jump to it, and shoot the target while airborne. Swing from the bar that drops down, and dash to the other side. Immediately turn around and shoot another target. Jump and dash back across the same gap to grab the page behind the unlocked door.
The last Codex entry is another one you can’t miss. Before your encounter with Mutated Seraphim, head up to the right of the room to pick it up.