- This level occupies the map slot MAP12. For other maps which occupy this slot, see Category:MAP12.
The level design is a multi-tiered hellish keep the Marine must climb and explore which contains the actual altar itself, within a bed of rock walls and a watery moat surrounding it. Horrifying decor litters the structure, from depictions of human sacrifice, burning red inverted crosses and a glowing, silver pentagram star in the altar room. This, combined with the stage's horrific music and constant thunder strikes, easily makes this the darkest and most frightening level in the entire game.
In order to find the secret exit you need to find a column with a soul sphere on it (the column is shaped like an arrow towards a wall). If you run into that wall you will end up in a cave with a Baron of Hell and the secret exit teleporter. The rest of the level consists of breaching a fort and fighting lots of demons along until you find the actual altar of pain and the exit.
It uses the music track "Lamentation Of The Forgotten" by Aubrey Hodges. The music for this level is the same music as PlayStation Doom's MAP21: Mt. Erebus and MAP36: The Crusher, which features babies crying and dark, ambient music. On Doom 64, it has been lowered to make the sound even creepier.
- Nightmare imp
- hell knights
- barons of hell
- Mancubi (Skill level I own Doom! or harder)
- When the level begins, turn around and enter the cave behind you. You will see a gap in the bottom of one of the walls here. Examine this wall to reveal a secret room and Soul sphere.
- (Exit to Secret Level 30: The Lair) Once you collect the yellow key, stop and look ahead. In front of you are the switch on the lower level, and a pillar with a Soul sphere. Make a dashing jump off the platform the yellow key was on, over the switch below, and stop on top of the Soul sphere platform. Turn on your automap, and you'll notice that the platform you are on is shaped like an arrow. If you run full speed off the platform in the direction it is pointing, you'll fly through a fake wall in the cliffside and wind up in a cave. After you kill the Baron in here, you will see an exit open up to the secret level.