Slayer doors require a Slayer key to open. They are usually located somewhere nearby and require a skillful platform to access them. These keys unlock extremely difficult battle arenas that always reward you with three weapon mod points. It's a great price if you can survive the onslaught. Below you'll find locations for the Slayer Key and all the other collectibles in Mission 2.
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How collectibles work in DOOM Eternal:
There are many types of collectibles in: Auto Cards, Runes, Praetorian Points, Weapon Mods, Albums, Cheat Codes, Toys, Sentinel Crystals, and Sentinel Batteries. Every level has an Automap, but not all levels will have Albums, Toys, Cheat Codes, or Slayer Keys.
Basically there are a ton of different collectibles, and below I'll list where to find them all - in the Fallen from start of level to end. Levels in are much more linear, so as long as you follow along, it's easy to get all of the collectibles.
Guide to collection sites
M2: Exultia - Collection locations:
Manuscript: After breaking through the first cracked green wall, look left at a dead end with this codex.
Rune: Found in the cylinder chamber after receiving the Blood Punch. Do not miss.
Toy - Arachnotron: In the same room, go down and look for a side passage with a cracked wall on the left. Clear your way to get this “?” Collector's item.
Manuscript: After the first red Demonic Infestation arena, you will have to climb a soft wall. On the wall, turn around and double jump to a ledge with this codex.
Manuscript: In the room where you will have to break three chains - the codex is right next to a chain.
Manuscript: Right, when you leave the king's chamber.
Sentinel battery: Found after the second Demonic Infestation arena. You can not miss it. We must progress.
Secret meeting: To the right of the Sentry Battery, use the double-jump and dashes to reach the poles and switch to the far tower. Use sticky bombs from your shotgun to quickly kill cacodemons.
Album – At Doom’s Gate (Doom): In hell, go down to the arm of the severed robot and hit the large crate. Aim for the side with the fist icon, then follow the underground path.
Automap: Inside the fleshy bowels where you will collect the energy source for the giant severed arm in hell. You can not miss it.
Poor Mod: Simply browse the Automap. Another collector's item not to be missed.
Manuscript: Blow through the wall with the giant arm cannon, then climb up the soft walls. At the top you will find this codex page.
Secret meeting: When you reach the giant laser lance held by the enormous Doomguy, descend below to an optional area with this secret.
Sentinel crystal: After using the Third Power Core to punch a hole through a giant demon, look for the crystal in the purple fluid on the left.
Manuscript: On the lower levels of the lava room where you discover the floating platforms that sink when you stand on them.
Slayer Key: After the killer door, enter the large chamber, then go back to powering up the dashboard recharge.
Slayer Gate: Found on the right, after completing the Falling Platforms Glove.
Toy - Cacodemon: In the lava cave where you will have to jump on each soft wall (and avoid the electric doors), there is a lava waterfall at the top. Double jump to the optional ledge. It's in the small corner.
Manuscript: To the left of the jumppad above the room where you meet the traitor.
