These three levels contain all of the collectibles you would expect from the main game, and earning them is especially important for two reasons. Finding all secret encounters and completing them will give you a new multiplayer skin that you can access from the customize menu. That's three skins in total for the first half of the DLC. Best of all, finishing runic gates will get new support rune powers and a new slot to make your Doom Slayer even stronger.
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Codex # 1: After getting the credit card, go down the stairs near the upper door. If you enter through the lower blue card door, continue straight ahead before going up the stairs.
Secret (1-Up): After the big fight in the big room of the oil rig with fire jets and turrets, you will have to blow a vent above. Before going down, look for an optional path with a vent going up. Jump to find the 1-Up. It's in the vents above the red key card room.
Secret meeting # 1: Take the elevator after fighting the Marauder, then turn right and look for a wall you can climb before jumping after the Spider to the left. At the bottom of a floor of the ledge, there is a secret meeting.
Codex # 2: Jump the ditch with Arachnotron and meet the two turrets and Cyber-Mancubus. Behind these there is a switch that you need to flip. Go up the stairs to the left to find this codex on a raised ledge.
Secret (Full HP + Armor): Ride the cargo container on the crane to a circular arena. There is a caged area to the left with the power-up inside.
Codex # 3: As you swing and use the wind turbine to launch yourself onto the scaffolding, look for a ledge to the right at the sealed door inside on the lower aisles.
Codex # 4: After meeting the Doom Hunter, you will enter the main facility. It is located behind the stairs.
Killers Gate: Beyond the point where the facility self-destructs, you will reach a combat encounter with a tyrant. Continue down the hallway full of tentacles and ignore the main path on the left. Continue forward and the killers door is on the right.
Loyalty card: Continue past the door of the killers. You will meet a tyrant in a small room - defeat him and the map console is right.
Killer key: In the map room, jump over the wall and look down - there are two poles you can swing on below. Swing on the poles and do a double jump to a vent.
Secret (+ Armor): In the podium room with the three turbines after the tentacle corridor, go to the room on the left at the top of the stairs. Smash the vent at the end.
Secret meeting # 2: In the podium room with the three turbines filled with water, just after the tentacle corridor, you will have to go to the office upstairs. Press the button inside, grab the environmental suit and swim down.
- In the water, swim to the left to find a passage leading to the secret encounter.
- You can also swim under the central turbine to find Munitions BFG.
Secret (2 in 1): This rare 2-Up is at the submarine base. When you have to fight the Tyrant, you will later dive into the water. When you reach your destination, you will see the 2-Up behind a locked door. Swim to the opposite end of the hall and exit to get it.
