The door to Chthon is the penultimate level in the first episode. It is rather small, but still filled with a few traps and enemies that will teleport into the map.
The starting platform is in a room, with one or two ogres on the ground level. There are a few exits, one is a dark tunnel, a lava pit leading to a gold key, and one that has a button in front of it. You will pass through this section a few times and may find it easy to clear out the ogres.
On hard difficulty: You can approach a lava pit and launch a grenade or rocket past the bars. This will activate the fiend waiting near the pressure plate, and allow you to avoid an ambush when you later open that door.
Silver key route
The corridor to the left starts with a button to open a door ahead. There's two ogres in this corridor. It will loop to a button, and a rocket launcher. If desired, you can get the secret for a quad damage, or save it until later.
The wall button opens the door to a larger room, showing yellow armor. Pressing the button in that room opens a floor panel that allows you to drop into a room.
After the drop, up to three ogres will lob grenades from the sides. Survive for a few seconds, and the floor will change into stairs, and into the spike wall corridor.
Soon after entering the corridor, one or two ogres will be released from thew all on the left, and you will need to quickly kill them. On hard difficulty, an ogre will also teleport at the end of the corridor when you advance far enough. When the ogres from the side are killed, the barrier near the end will open, allowing you to escape.
This returns you to the previous room, where you can finish off the ogres, and collect the silver key. This will also illuminate the darkened corridor.
The next step is the gold key, which is across a lava pit. If you can't reach the gold key, you will need to go through the dark corridor, which is now lit after collecting the silver key. The dark corridor simply shows a narrow twisting bridge over a pit that shoots spikes at those who fall in.
After you cross a bridge, an ogre will emerge from an alcove. The doors to the left lead to a large chamber, which also has another enemy ready to attack. This chamger has a button on the right that will raise the bridge to the silver key door, and drop several zombies from the ceiling.
Open the silver key door. On hard, a fiend will jump and attack. Press the floor button to extend the bridge leading to the gold key, and also open the teleport gate leading back to the starting area.
Return to the starting area, and cross the bridge. When you get the gold key, either a fiend or shambler will teleport in and attack.
Enter the gold key gate, where up to two ogres wait. Press the button on the left to open the door.
In the next room, the floor button will open the door ahead. It is initially dark, but will light up and reveal a shambler. The level will end when you drop into the teleporter at the end of the corridor.
- At the start of the map, you can rocket jump to the gold key, activating either the fiend or shambler. This allows bypassing most of the level.
- To a lesser extent, you can skip the spike wall with a rocket jump to the silver key.
There are four secrets on this map. Players that launch this map directly may find it difficult to complete the level without using some of them.
- At the first wall button, turn around and look up. There is a shootable button high above, which opens an elevator. Right the elevator up, and drop onto the platform to collect a quad damage.
- In the spike wall trap, there is a teleporter that is behind the spike wall. You can reach it after collecting the silver key, and waiting for the bars to open. This leads to a super nailgon but unleashes three scrags. From the super nailgun, you can also perform a jump to the platforms on the right for a megahealth.
- After starting to raise the bridge to the silver door, a button should become active in the middle of the room. Shoot this to lower the staircase to a set of rockets.
- From the above secret, wait for the stairs to return to the prior position. This reveals a teleporter that leads to a megahealth.