The first mission called Hell on Earth in Doom Eternal contains a total of 12 collectibles. There are 6 codices, 3 modbots and 3 toys.
Barge (Travel Point: Hell Priest Barge)
Toy: Zombie (Earth)
From the starting point, exit the room and head down three flights of stairs. At the base of the stairs, follow the path left through the center of the room, passing an explosive barrel and cages filled with bodies. When you reach the back left corner of the room, there will be a cracked section of wall next to one of the cages. Melee the wall to create an opening then follow the path forward to locate the toy doll at the end.
Modbot: Weapon Mod
From the starting point, proceed forward a short way until you reach the section where you have to melee the giant cube. After you melee the cube, use it to reach a higher level to find the weapon mod near a doorway leading deeper into the area.
Codex: Story of Hell – Hell Barges
After collecting the Weapon Mod, you’ll pass through a narrow hallway leading to a big open room filled with enemies. Defeat all the enemies to unlock the way forward. The book can be found in the middle of the hallway leading to Deag Nilox’s Chamber.
City (Midtown)
Codex: Story of Earth – Remaining Human Populations – Part 1
After exiting the Hell Barge, follow the highway to locate the book near the entrance to a ruined building.
Modbot: Weapon Mod
From the above collectible, jump into the building to find the weapon mod near the entrance. (See picture above.)
Toy: Doom Slayer
From the previous weapon mod, head further into the room to find a crack in the wall. Melee the wall then head through the opening and jump up to the ledge above next to an impaled corpse. Head past the corpse to find the toy tucked away at the end of a short hallway.
City (Travel Point: Ravaged Cathedral)
Toy: Imp
This collectible is located in the building with a sign boldly saying “SCOTT” in neon lights. To reach it, you will have to climb on the overturned semi-truck across from the SCOTT building. Follow it to the end then jump to the ledge above the ShuZ sign. Follow the path straight ahead to come to a crack in the wall at the end on the left then melee the wall. Drop down the hole in the floor to find the toy.
Codex: Story of Earth – Remaining Human Population – Part 2
Head through the doorway of the building with the SCOTT sign, passing the UAC Spokesperson hologram. Make your way across the room towards the green light on the ceiling, passing through the door under it. Climb up a ledge following the path right to the broken section of the windows but don’t go out it. Facing the broken window, turn left to see another section of floor down a hallway. Use the double jump to reach that area to find the book.
Modbot: Weapon Mod
This weapon mod is located in the underground section of the subway just past the three fire shooting tubes.
City (Travel Point: Metro Station)
Codex: Story of Earth – Formation of the Arc
After you jump across a large chasm and head through a subway train, you’ll end up in a subway tunnel. The book will be located straight ahead in the tunnel just past a door leading to a maintenance room.
Citadel (Travel Point: Citadel of Hell)
Codex: Story of Hell – Dreag Nilox
When you enter the Citadel, take the stairs to the right of the statue and follow the way forward until you come to a big open room. In the room, take a right heading up a flight of stairs. You will find the book on the right side of the room.
Codex: Story of Hell – Hell Priests
From the above book, head past the flaming statue towards the middle of the room and drop down near the skull pedestal. The book will be located in an alcove behind the pedestal that is located in the middle of the room.