After the dialogue at the beginning, head along the path. You will run into howlers as you go, which are imp-like enemies that fire fireballs in quick succession when you stay in their line of sight. They take a few pistol shots to kill as they have 80 hit points, a bit more than an imp. As you go, be sure to pick up ammo clips to replenish your ammo. Some supplies are on the ruined structures, so you need to jump up to reach them. Get a shotgun as well as some armor from the shotgun guys in the shack along the way. Look out for mauler demons on the way out. They behave similarly to demons, but lunge at you when they can and they have 150 hit points, a bit more than a regular demon. When the path verges westward, you will see a closed gateway and a couple of soul harvesters along the wall. These are imp-like enemies that fire homing projectiles, so kill them when you can. They have 120 hit points so they are twice as strong as normal imps. To open the gateway, there is a nearby shack. You need to jump into the shack through the roof, as the door cannot be opened from outside. There is a stepping stone to the east of the shack which you can use. Once inside, press on the large switch, and then open the door to head back outside.
Mauler demons come pouring out of the gate so kill them quickly. In the ruins ahead, press the large satyr switch, to enter a part of the ruins, killing shotgun guys until you find another large satyr switch. This releases some satyrs. These enemies have 300 hit points, exactly like the revenant, but only a melee attack. The bars blocking off the western exit of the ruins will be open, so go through to find the entrance to a large temple. A whole mess of howlers and soul harvesters guard the entrance, so take them out. Once you do that, you need to figure out how to open the entrance. Two platforms which had soul harvesters on them have plates you can step on. You need to step on both of them. When you do, kill the soul harvesters that appear, and then the temple opens up. When you enter, grab the backpack and the megasphere, and satyrs and hell warriors. Kill all of them; note that the hell warriors are slightly stronger than the Hell knight in strength but can use their shields to nullify damage. Once they are all killed, the wall opens. Grab the ammo around the smoky cauldron and then jump in.
Upon jumping in, you will find yourself in a cage, and five cacodemons come up. You need to kill them all, and then you will teleport into a prison cell. Other marines are in the other cells, and Hell knights will eventually spawn in. Help the marines out by killing the knights, and then more Hell knights appear in your cell. The cell doors should open by then, get out and then follow the hallway, ignoring the doors for now as they cannot be opened yet. On the other side, press the skull switches and enter the prison cells for supplies, but you cannot enter the middle cell where the yellow skull key is yet. Back in the hall, press the wall north of the cells to lower it. In the cave that follows it, find a skull switch somewhere within, which opens up a few halls, with satyrs and howlers abound. Find the path leading south to a wooden door, and kill the Hell knight on the other side. Climb the steps up to find yourself in a room with soul harvesters on balconies and satyrs on the floor. Stay on the balcony you are on and head for the other door on this level. Inside is a Hell knight and some steps leading up to a lion switch. After pressing it, return to the previous room to find cacodemons coming out of the stained glass window. Get rid of them to have the door open.
The way to the next area should be open as well. This place resembles an underground city block, where mauler demons patrol the streets and Hell knights are in the central plaza. Soul harvesters will also be guarding the blue skull doors to the east. You can enter the northeastern building here (Thorn's House), and go through until you find a large satyr switch. Look out for the baron of Hell that appears behind you as you press it, then head back outside. There are three mini-paths that are open. On the eastern side of Thorn's House, you can grab the medikit inside a small room to lower a wall with pain elementals and a super shotgun. The other two paths, closer to the entrance to Thorn's House, lead to two skull switches. One opens up the entrance to the southwestern building (Belphegor's Bloodbath), and the other opens a shack south of that building with a soul sphere. Some satyrs will pour out of Belphegor's Bloodbath, so kill them and enter. Barons of Hell will be in the pool, along with a megaarmor. The door inside has an arch-vile and a skull switch. Pressing this one opens the northwestern building in the complex (Skull Tavern). Satyrs and soul harvesters populate this area. The second floor of the tavern has some rooms with barons and supplies on them, as does the first floor, but the rightmost room on the first floor has a skull switch opening the leftmost room with an arch-vile inside.
Press the switch in this room to lower a lift down into the catacombs. Here you will navigate until you find skull switches which open up more of the catacombs. There are various monsters in this area, including nightmares. These things are hard to hit; they materialize when they get into melee range, so shoot quickly, but remember that you cannot see or kill them if they are at a distance. They have 100 hit points, a bit less than a demon. When you find a metal lift, use the nearby switch to ride up. The large satyr switch at the very end of the hall opens up the eastern half of the catacombs, while the other satyr switches open up walls leading to the prison cells you were at earlier. Return to the catacombs and navigate your way through the eastern side, looking out for nightmares. The skull switch in the middle of this area lowers walls with revenants and an arch-vile. The northern wall reveals a lift to ride up, leading to a similar hallway to the other one. This time, the satyr switch at the end of the hall does not work, although you can still press the other ones to open the back routes to the prison cells. You should remember the yellow skull key was in the middle one, so open the way to it. Be careful not to fall into the lava as you pick up the yellow key.
Hell knights spawn in once you pick it up, so kill them and return to the underground city complex. At the southern end of the complex you will find a door with yellow skull markings around it. Press the switch next to the door and it will eventually open. Press the skull switch inside to ride up, killing the shotgun guys on the next floor. There are hell warriors and satyrs here, along with cacodemons floating above the hellscape. You will see the blue skull key up ahead with floating platforms. Jump to it to be stopped by a ghost marine. After some dialogue about an upcoming pentagram and how to deactivate it, he teleports away and you also teleport into a small house.
There are no enemies in this house, but there are a few pickups to collect if you need health. Head out to the patio outside the house for a chainsaw; you will then be prompted to hit a switch in the computer room. Go there and find a switch on the southwestern wall. This opens up the room with the BFG9000. The ghost marine will appear, and say some closing dialogue before you teleport back. When you return, there will be two arch-viles attacking, so get rid of them and return to the city complex.
The southeastern part of the city complex is now open, with mauler demons and satyrs attacking. Go to the central pillar to find four satyr switches. The southern one opens two doors to the north. Kill the arch-viles in here and press the switch to open up the southern shack with hell warriors and an armor. When you are ready, open any of the blue doors to the east and kill the Hell knights, soul harvesters, and baron of Hell, then take the lift down to the hellscape below. In this large cavern, get rid of all the monsters you can and head for the ruined shack to the southeast. Jump on the block and jump your way into the shack. Defeat the hell warrior here and press the satyr switch to open the way out. Now make your way to the northeastern shack, looking out for barons of Hell in one shack east as well (there is a soul sphere there if you need it). The red skull key is in the northeast shack, guarded by a baron of Hell and an arch-vile. Grab the key and head into the middle of the hellish cavern where a red switch is. Press it and kill the two arch-viles that appear nearby to open the door.
When inside, after the greeting dialogue, enter a wooden door and go up to a plate you can step on. Now go back and enter the other wooden door. An arch-vile and some hell warriors will appear, so dispose of them and look out towards the large pentagram. As the ghost marine had said, you should not enter the pentagram when it is active, but you can see a skull switch directly across the pentagram. Shoot it to lower some walls behind you. These walls lead to symmetrically identical hallways. Suppose you are taking the left hand side first. Kill the baron past the door and climb the steps. Open either door leading south and open the next door past the Hell knights. Press the switch on the plate to solve one-fifth of the puzzle, and look out for the arch-vile that emerges beyond the painting. Return to the hub before the pentagram and take the right side, doing the same thing until you find the second switch near a plate. This solves another fifth of the puzzle, releasing another arch-vile. Go back to the hall and take the northern door, where the next door should be open. Repeat the same procedure and you have three-fifths of the puzzle solved. Now go back to the left hall and the northern door is open. Press the fourth switch and then head out as usual, but there will be a teleporter along the way which you can access.
Take this teleporter, kill the hell warriors in the next room, then press the final switch. This completely deactivates the pentagram, so now you are able to enter it without repercussion. After killing the arch-vile, the door opens, so return to the hub before the pentagram and enter. You will now face Cheogsh, a fiery, baron-like monster. His attacks include shooting a spread of smaller fireballs along the ground towards you or a one-two fireball attack. He also creates a spread of explosive fireballs that careen along the ground towards you, while teleporting to one of the pads. Cheogsh has 10000 hit points, roughly three times more than a cyberdemon, so he will take a while to beat. When you defeat him, the wall to the north opens, so press the skull switch to call a lift down. Use the skull switch inside the lift to ride up, and then take either door to appear back at the temple entrance. Go outside to see that the sea is now blue and the sky is not red anymore. Return all the way back to your starting point to leave the level.
Other points of interest
- Before the large temple entrance, look northward for some sticks with heads on them. Next to the triplet of heads on the right, hop the blocks and jump in the small pit for a radiation suit. (sector 415)
- Climb the steps in Secret #1, but this time jump up to the top of the gate that barred access to the temple entrance. Move across and follow the armor bonuses until you find yourself on top of the closed ruins you were in before the temple entrance. Drop down into a sector containing a box of rockets, an armor bonus, a stimpack, and an ammo clip. (sector 423)
- In the cavern with satyrs and howlers north of the prison cells, after you press the skull switch, head to the westernmost part of the small cavern. On the inner bend, press the wall to find two rockets and a partial invisibility. (sector 926)
- The stained glass window in the room with the balconies (the room is surrounded by the prison cells) has cacodemons inside that you must kill to progress through the level. After killing them, you can go up to the third floor and jump through the stained glass window to find cell charge packs and a stimpack. (sector 796)
- The BFG9000 room counts as a secret. (sector 1763)
- Press the central switch at the western part of the catacombs under the city complex (linedef 10517). Return to the northern end of the western catacombs to find a soul sphere. (sector 234)
- When you are teleported to the small house where you get the BFG, open the closet in the first room next to the mirror for an armor. (sector 1998)
- The central pillar in the southeastern part of the city complex has four satyr faces. Press on the northern face and run to the fountain at the center of the city complex for a berserk pack. (sector 953)
- After deactivating the pentagram, head back through the right halls before the pentagram to find a teleporter similar to the one you entered to press the final skull switch that deactivates the pentagram. Through this teleporter you will find loads of supplies that will help for the boss. (sector 3894)
Areas / screenshots
This level contains the following numbers of things per skill level: