This level occupies the map slot MAP21. For other maps which occupy this slot, see Category:MAP21.
MAP21: Twilight Lab is the twenty-first level of Memento Mori. It was designed by Andy Badorek and uses the music track "Mummy Rags" by Mark Klem. This level was first created by Badorek in 1995 as a standalone level, TWILAB.WAD; the Memento Mori version features several modifications for cooperative multiplayer gameplay.
Upon going down the elevator from the start area, head near the first red door. In the corner opposite of this door is a section of wall covered with a spray-painted X. Open this wall to reveal a hidden room and then walk across the north of the rock pit to reveal a secret compartment (sector 32) containing three lost souls, a plasma gun(, a bulk cell on easy and medium skill levels) and two energy cells.
Enter the first red door mentioned in secret #1 and then the northernmost door. Lower the dark metal lift and ride it up to encounter four sergeants and a chaingunner. Open the dark metal wall in the east to enter a room called Central Computing as indicated on the signpost nearby the first red door. This is a room with a walkway (and some sergeants) along the edges; drop into the pit and press on the inner sides of the walkway to lower it like an elevator. This will reveal an ambush with cacodemons (and pain elementals on hard skills) and two holes in the northeastern corner of the room. Enter either one of these to find a secret alcove (sector 125) with two lost souls, two sergeants, (skill dependent) none/two/four chaingunners and a berserk pack. The Central Computing room can also be accessed by exiting secret #1 into the hallway, entering either door in the northern wall, approaching either health bonus in the east to ride up a lift, reaching the east of the red floor maze to ride down a lift and proceeding through the passage there.
Upon entering the Toxin Experiment area (through the first red door, then the door right east of it), pass the hallway, ascend either stairway and flip the switch to lower the imps' alcoves in the north of the hallway, revealing a hidden alcove (sector 398) between those with three rockets and a stimpack.
In the Containment Area (crate storage) is a computer console (sector 225) upon which three health bonuses and two armor bonuses rest. Head over to the south of the room and ascend the stairs building crates to encounter some sergeants and to reach the room's southeastern part. Drop down on the floor to the east and follow the passage to kill a demon. Approach him, walking over the small crates so a pillar will lower like an elevator, revealing a chaingunner. Take this elevator up, jump onto the brown crates (with a health bonus) to the west, and then jump over onto the gray crates to the north (with two barrels, (a chaingunner on medium and hard,) a sergeant and an imp). (Return to this point to access non-offcial secret #3.) From there, jump west across the gaps to the brown crates (sergeant and barrel) in that direction, and then the gray crates (imp). Jump across the next gap to the gray crate (health bonus) and then navigate the gray crates over to the northwest. Head over to the eastern edge of the last crate stack, and then jump on top of the console to register the secret.
In the gray stone room with the toxic pool (behind the blue door west of the elevator you rode down from the start area), there is one patch of wall in the south that is green. Open this wall and the one after it to access the brown sludge pit in the southern room. Behind the mechanical structure is a secret alcove (sector 322) containing a bulk cell and a megasphere. To exit the pit, take the elevator in the west of the alcove, and then lower the railing to the west.
From the switch mentioned in official secret #3, proceed to the east, pass the door to the south and follow the walkway. Before descending the stairs, open the differently textured part of the northern wall to find a compartment containing a chainsaw.
After exiting non-official secret #1 and descending the stairs to the west, push the metal wall to your right to lower a lift leading to the the imps' alcoves in the south of official secret #3's hallway where you find a rocket and three health bonuses.
From the point (the first gray crates) mentioned in official secret #4, drop down on the brown crates to the north. Follow the walkway built by these around the corner to the right, jumping over a gap to find a computer map.