All the collectibles in chapter 18 of Doom: The Dark Ages (2025)

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Chapter 18 of Doom: The Dark Ages is fleshy and gloriously difficult

Secret area 1, Demonic Essence, and a gold chest

Secret area 2 and a Life Sigil

Secret area 3, gold bars, and gold stacks

Secret area 4 and a gold chest

Secret area 5 and collectible toy

Secret area 6 and a Life Sigil

Codex Page for Commander Thira

Chapter 18: Belly of the Beast in Doom: The Dark Ages has the Slayer navigating the insides of the Old One, after the ancient deity rises up to ingest him in the previous chapter. In a stark contrast to the first half of the game, the latter chapters are set in a Lovecraftian realm, and you can read in our review of the game how impressed it left us.

In order to get 100% completion, make sure you complete the mission challenges for this level.

Chapter 18’s mission challenges are-

Completing the optional encounter in the stomach
Parrying Cosmic Barons 10 times

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Chapter 18 of Doom: The Dark Ages is fleshy and gloriously difficult

Is this how I meet the Old One?

In chapter 18, the Slayer is now inside the Old One — the ancient deity he jailbroke in Chapter 14. Following the objectives in the chapter along with the instructions provided below, it won't be long before you can claim that sweet 100% completion tag for the level.

Path tracing hasn’t arrived in the game yet, but the Lovecraftian horrors are a spectacle to behold, thanks to the id Tech engine 8.

This guide covers each collectible required for 100% level completion in chronological order. It is recommended to follow this guide in the order stated.

Secret area 1, Demonic Essence, and a gold chest

A Cosmic Baron and multiple tough demons

All the collectibles in chapter 18 of Doom: The Dark Ages (2)

The first encounter in the chapter is unmissable. You’ll fight all sorts of demons before a Cacodemon Hybrid, a Cosmic Baron, and an Acolyte spawn. This is a great place to constantly parry the Cosmic Baron and progress or wrap up the Toe-to-Toe mission challenge early on. Then, take on the Acolyte to get a Demonic Essence for maximum armor.

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Interact with the fleshy, objective organ in the center of this arena, and a small cutscene will play.

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Then, without moving, look directly to your left. There’s a small hole in the fleshy wall.

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Approach it, and you’ll unlock secret area 1 and a gold chest.

Secret area 2 and a Life Sigil

A skeleton peering through the flesh

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Odds are that you’d lose a Life Sigil or two in the fight you just finished. So, walk back to the center of the arena where you just punched into the green, fleshy organ.

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Then, look right past it to see a green flesh node peeking from the wall.

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Approach it and use it with your Shield Recall Jump.

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You’ll land in secret area 2 and there’s a Life Sigil at the end of the trail. Proceed with the objective using the blue energy node.

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Secret area 3, gold bars, and gold stacks

Tons of big block bashing for the Slayer

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In the very next area, you’ll find yourself in this location on the map.

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You’ll notice 4 Shield Bash symbols on four fleshy blocks here.

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Start from your left. Bash into the first one on the left, and some demons will appear.

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Dispatch them, and head into the block you just bashed to pick up gold bars.

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Then, do the same with the next, but there’s no reward here. Shield Charge into the third fleshy block from the left, and you’ll have to fight a lot of Shielded Soldiers and a Cosmic Baron.

You can parry its attacks to progress through the Toe-to-Toe mission challenge if you haven’t finished it yet.

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Once the enemies are down, look inside the block you just bashed into, and you’ll find two gold stacks. You don’t need to bash into the fourth box, since it has no reward and will only spawn tentacles all across the floor.

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Next, look up to see a green flesh node and a blue energy node adjacent to each other.

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Throw your shield into the energy node and wait for the flesh node to reach the ledge up above. Quickly, use the green flesh node to get up above, before the entire mechanism goes back to its original position.

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Then, head up the scramble wall, and immediately turn around, facing the opposite direction from the objective.

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You’ll see a long, fleshy tunnel stuck to the roof.

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Head into it and keep following it to unlock secret area 3 and the only Red Ruby in the chapter. Use the energy and flesh nodes overhead to get back up the scramble wall above and move into the next arena.

Secret area 4 and a gold chest

Small detour for a crate of gold

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In the next arena, you’ll find yourself facing off against a Revenant and an Agaddon Hunter.

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Dispatch them, and head towards the objective here. Before you punch into the green button/node, look to the right, and you’ll see a green flesh node overhead.

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Use it to get up to its platform, and walk along it to unlock secret area 4 and a gold chest. Then, finish up the objective here and move to the next one, right behind the Sentinel Shrine.

Secret area 5 and collectible toy

Thrown into a poisonous crucible

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In the next arena, the Slayer has a lot of slaying to do. Keep fighting through the long arena, taking down enhanced demons of all sorts.

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Finally, you'll fight a Vagary and a Revenant at the end, and here's where you'll be on the map.

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Then, turn around and start walking back the way you came, sticking to the left wall of the tunnel.

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Here, you’ll come upon a blue energy node to stick your shield into.

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Do that, walk inside, and pick up a trail of gold bars.

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Keep heading forward and drop down the hole in the floor.

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You'll fall into a small, tight arena with poison surrounding it. This is secret area 5.

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Fight through the multiple demons that will appear here, and you’ll finally be able to get the secret collectible toy for the Komodo. You’ll also finish the Swallowed Whole mission challenge. Head through the red portal.

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Secret area 6 and a Life Sigil

Took me a while to notice the solution hanging above my head

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You’ll find yourself in this area after exiting the red portal. Dispatch the Shield Bearer in front of you, and start heading right.

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Reach the end of the tunnel where you fought the Vagary and the Revenant. Here’s where you’ll be on the map.

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Then, while facing the barricading vines overhead, look to the right to see a hole in the wall.

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Enter through it, and you’ll be met by a moving, fleshy, and toothy tunnel. You can’t get through it unless you hit the blue energy node directly overhead on the roof. Use it and the tunnel will stop moving, giving you safe passage.

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You’ll come upon a long trail of gold bars, ending in a scramble wall.

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Climb it up, and you’ll unlock secret area 6 and a Life Sigil. Bash through the wall ahead to land back where you started.

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Codex Page for Commander Thira

Knowledge about King Novik's only daughter

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Now, stand next to the third fleshy node/button the Slayer needs to punch through. This is where you’ll be standing on the map.

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Look past the objective, and you’ll see a small stream of poison, leading to a Codex Page.

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Grab it while jumping and landing down below to avoid any poison damage.

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Get back and hit the third green node, and the locked door behind it will open.

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Pick up the final trail of gold bars just ahead, and go through the rest of the chapter. While the collectibles and secrets are all done, the Slayer still has a lot of fighting to do. Rip and tear.

All collectibles in Chapter 18: Belly of the Beast in Doom: The Dark Ages

Chapter 18 is a disappointing chapter because of the way Cthulhu was handled.

Once that’s done, Chapter 18 of Doom: The Dark Ages will be complete, and you’ll finally have the 100% completion achievement for this level. It's a disappointing chapter because of the way Cthulhu was handled — the Slayer does cause the Old One's demise, but it's so infuriatingly anti-climactic.