Doom: The Dark Ages Chapter 14 is a challenging mission that requires attention to detail. To help you navigate the level, we’ve compiled a list of all the secrets and collectibles in chronological order.
The first secret is a gold chest located on the left side of the first arena, where you kill the komodo demon. To reach it, follow these steps: go up the hill above the cave, drop off the edge onto a ledge that leads to the back of the cave.
After gaining access to your dragon, follow the objective marker to the next zone. In this area, head towards the big building at the back of the area, past the Sentinel Shrine, and you’ll find a cave under the right side of the structure. Inside, you’ll find another gold chest.
Next, head into the shipwreck and follow the objective marker through the ship. To find the next secret, hug the left wall and break the wooden barricade to reach an Arachnotron action figure on the main deck. Then, head up to the helm of the ship and break the chain holding a cube in air to access another gold chest.
In this area, you’ll also find the Spire of Nerathul codex collectible at the end of the shipwreck.
To unlock the final secret, activate all three stasis emitters. After that, fly through the tunnel leading outside of the map boundaries. You’ll find a secret landing zone with hordes of demons to cleave through, containing both a wraithstone and a Nightmare Grenade Launcher weapon skin.
Finally, head down the stairs on the final landing pad, climb up the wall to a tower, use the jump pad to launch yourself to the top, then leap off onto an island below. There, you’ll find the last secret collectible, the Serrat codex entry.
Source: https://www.pcgamer.com/games/fps/doom-the-dark-ages-spire-of-nerathul-secrets-collectibles-chapter-14