Several things to mention about this guide:

1. We will talk about the entities that we meet along the way
   
2. We will talk about the events that happen on the map with no player input
   
3. We will touch on events that the player has to participate in in order to activate them

Not everything I say or theorise on is probably 100% true, thus I will gladly fix any mistakes that I might (and probably will) make in writing this guide. We will try to touch on everything that I have experienced and noticed so far.

Most knowledge I have learned from watching notandum's exploration videos, which pretty much gave me a reason to explore the map for myself, and to recommend its exploration to some of my friends.

Also, most names of locations, events, and NPCs are of my own creation, so they will probably not be the same as the official ones, sorry for that. (for example I call "Submarine Room" the "Pool Room")

Obviously, massive spoilers ahead.

todo: add images for everything

Here is what I did to pretty much beat the map.

You first need to grab the Doll from the Easter egg room, and bring it to the Dark Room.
You then will see that inside it, there is the Chair, on which You have to sit the Doll.
The Chair and the Doll will start spinning, while chaotic music plays.
Shortly after that, you will start falling into the Abyss along with the Chair.
While in the Abyss, you now have to destroy the Chair.
Once you have destroyed it, the Doll will tell you that "This is not how this is supposed to end.", and it probably really is not, but that's how I have progressed so far.

After you have been teleported onto the map surface again, you should now wait until the Chair appears again inside the Dark Room. Once it does, you will have to destroy it with a grenade explosion, which will spawn a bunch of Stalker NPCs shortly after. Eventually they will all catch fire, and your job now is to wait until one remains -- that's Ratman, and then you'll have to follow him to his room, which is inside the Pool Room.

That is one of our main places of operation on this map.

From here you should take the Radio, tune it to Portal radio loop station, and go visit as many Hidden Transmission locations as you can find, finishing with placing it on top of the tall building near the water.

When the Radio has finished exploding, you should be able to complete the Altar for the first time, and while you're searching for the props that you need for the Altar (full list of which i have inside the "Altar" section), keep an eye for the Woman NPC that you have to block in order to gain the Curse Detector.

Assuming you now have the Detector, you can finally start killing bosses and upgrading it.
Also it's worth visiting the Vent sometimes, it tells you where to go and what to do most of the time.

You can take the Detector to the ARC entrance and look around the building, just so the game remembers that you've went there.

I assume the progression ends once you upgrade your Detector three times, killing three different Grigori bosses spawned from the Altar, because after the third one the Vent stopped working entirely, no scribbles, no music. Not even any more props for the Altar, the map was over pretty much, though non-player stuff kept spawning and appearing.

I will call it the end of the map here, however.

A series of long concrete tunnels, that connect different parts of the map, and that also have some very interesting anomalies happening inside them.

Sometimes the hallways extend themselves in unexpected manners, leading into themselves, and bringing you into places where the travelled path makes no sense.

The image used here is from the official gm_construct 13 beta addon page.
If it says "Invalid URL" just click on the actual link on top of that window.

The same Mirror room as it is in default gm_construct, but with a twist.

Not only does it connect to a tunnel on other side, but you can also walk through the mirror and travel into a mirrored version of the map! As far as I could tell there's no new content inside the mirror, everything stays exactly the same, except mirrored.

upd. 14/09/2022

It seems that after travelling to the mirror world, the game starts spawning hints and various text all over the map, that you can see exclusively when mirrored. For example:

• It marks all of the spawn places of items needed for the altar with X marks
  
• There is now text on a wall in the room near the mirror room, that says the following: "YES! Prohibited materials ahead", "DO NOT OPEN", a symbol of some kind of a gyroscope.
  
• Next to it it says "ANCIENT EVIL INSIDE, DO NOT OPEN"

Left to that text you can indeed break a part of the wall open, but nothing seems to happen from what I've seen so far.

Near the A.R.C. entrance it says "SOMETHING DEEP DOWN BELOW". Technically, The Limbo is there, but not sure how that plays in.

In Ratman's Lair, it says "Receiving from the unknown" with a doodle of The Radio. Also, inside, there are several X marks on the floor in various places.

A staple of Garry's Mod horror and ARG's, The Dark Room is a pretty big unlit empty room, that houses several events and encounters on this map.

Any light sources rapidly dim and become useless when placed inside The Dark Room.

If using mat_fullbright 1, getting close to the Room entrance will automatically revert it to 0 for every frame.

Spawning your own NPCs inside also doesn't usually end well. Spawned NPCs were found to be:

• Turned into ragdolls and dragged across the floor
  
• Slowly phased out of existence
  
• Slowly phased out of existence spawning a hostile Stalker NPC afterwards
  
• Running away outside the room

Going to the corner where Grigori spawns teleports the player to a different corner near the main entrance, but only does that when the Player looks at the floor of Grigori's corner.

A huge unlit empty room that you fall into when sitting the Doll onto the Chair in the beginning.

So far no idea what role it plays in the map's progression and lore, but the Oracle Vent was spotted showing me an image of the vertical tunnel that leads from The Dark Room into The Abyss, crossed out with a red X, with text "I NEED IT?" drawn nearby, so maybe you have to not fall there ever??? No idea, honestly.

Just like The Dark Room, resets mat_fullbright to 0 on every frame, is also completely invisible when noclipping out of bounds.

This is your go-to room for hints in case you ever get lost.

A small room inside the big building near the hangar. Entrance is located left of the lambda sign that is painted on one of the walls of the building's interior, and can be broken with any weapon.

After entering the vent, the player has a choice of either crawling towards the Oracle Vent or the player can crawl towards The Dark Room and exit there, if he or she really wants to.

After reaching the actual Oracle Vent room, the wall should usually tell the player what he can/should do now to progress, but sometimes it simply displays some of the map's anomalies, like the Gatekeeper near the Dark Room main entrance.

After assembling the third Altar and killing the third Grigori boss the wall seems to stop working entirely. Still reacts to the Radio being brought to the wall however.

A room where Ratman lives.

Has the basic collection of things for basic human needs: a bed, a table, a chair, and a refrigerator.
A place where you will be a frequent guest.

An entire building, made out of HL2 and stock Phoenix building props, with an "Anomaly Research Center" title behind the chair inside the office room.

Can be accessed using the Curse Detector by placing it near the back entrance of the Dark Room. The Curse Detector will shine blue when you open the portal to The ARC.

The location will not allow the Player to noclip outside of its bounds, though with some experimentation, it was found that the entire interior, which is made out of props, is located somewhere inside of the Abyss, which is probably why it doesn't want the player to go outside its boundaries.

The location seems pretty useless so far.

This specific doll seems to be one of the main elements of this map, as we do need it at least twice to trigger an objective.

First time, for The Chair objective, and second time for the altar, though the map spawns a new white one for us.

The Chair spawns in The Dark Room, and has several behaviours.

The behaviours I have experienced so far:

• It can spawn looking in a random direction.
  
• If you bring The Doll, it will spawn looking directly at it.
  
• It will start spinning with The Doll, if you sit The Doll on top of it.

The only Stalker NPC that survives after the Chair explosion, and the one that leads you to his Home inside the Pool room.

Will become hostile you if you destroy his furniture too often.

This is your main companion on this map.
A traffic cone with a light on top of itself.
The light goes on whenever you bring it close to any anomalous places.
Can also be upgraded into a healing station using Food Rations.

Ration upgrades list:

• First Ration Upgrade makes it heal you slowly.
  
• Second Ration Upgrade makes it heal you more rapidly.
  
• Third Ration Upgrade makes it able to give you armour along with HP.

If you ever lose the Detector, you can teleport it to yourself using this command:
ent_teleport gm13_sent_curse_detector

This is your second companion on this map.
The Radio is given to the player after discovering Ratman's Room.
It is required to progress, and it is required to listen to hidden transmissions.

To listen to hidden transmissions you have to press E on the Radio until it starts playing the Portal radio loop, and then you have to place the Radio back to back to the point of interest (usually a big black spot on the wall), the music should change, and then things should start happening afterwards.

Also, if you happen to stumble upon a Half-Life 2 creepy trumpet transmission, you can enter the numbers the woman tells you into the console, and you will be teleported to the corner of the Dark Room to Father Grigori, which will ask for your help defending against fast zombies.

Two of them.

There's a "sad Grigori", that stays in the corner of the Dark Room, and there's the Boss Grigori that kicks ass when you assemble an altar.

A group of 1-6 fake players with Kleiner playermodels appear in the world, and start causing damage.
When they appear, Physgun will start leaving "untextured" decals on everything it's hitting, so it's impossible to miss.

They will randomly shine their Physgun beams, they will noclip across the map, and they will damage and kill any player that gets too close to them. They can also grab players with Physgun beams, which will also kill the player. And they also spawn crates, that eventually explode after Mingebags disappear.

A fully black Kleiner NPC, that will walk away and turn transparent when he and the player see one another.

If too close to him, he will shriek, and the Player will be very rapidly teleported across the map and will eventually be killed inside The Dark Room with a meaty slap, all while a creepy ambience plays on background.

A grey static statue of Gordon Freeman. He spawns randomly around the map, tracks player's position for around 3 seconds, and then freezes. He is mostly harmless.

If you put the Curse Detector too near him, he will take it with him. But he will soon return and bring it back.

A model of info_player_start, that appears on spawn points of the map.
When you try walking into him, he simply teleports into a different spawn position, and after 5 or so teleports he simply disappears. Doesn't seem to do anything else.

A collection of props that sometimes spawns in one of the several locations.
The collection usually consists of one chair, one Lamar cage, one tub, one wrench, and one cardboard box, that seemed to burn indefinitely when we found it. It also seems like it's burning only in two out of six locations.

Spawn locations found so far:

• On top of the entrance to the Dark Room
  
• In Ratman's Lair
  
• One first floor of the furthest building
  
• Near the entrance to the tunnel, back of the Dark Room
  
• In the room near the Mirror Room
  
• On top floor of the building near spawn

The altar cosists of:

• Ratman's table
  
• Five baby dolls*
  
• One skull*
  
• One oversized doll*

A fully assembled altar starts a Grigori Boss Battle.

Locations of these props:

• The lampshade inside Ratman's Room
  
• The bookcase inside Ratman's Room
  
• A trash bin inside the Easter egg room
  
• A globe on top of the building left of spawn
  
• A globe on a balcony of the building left of spawn
  
• A globe on top of stairs near the Mirror roam, near the tunnel
  
• A globe on top of the building near water

*which are scattered across the map in forms of other props

The Father Grigori boss battle has several levels of difficulty, getting more difficult with each altar that you build.

Father Grigori wields a shotgun, and does quite a bit of damage, but nothing too spectacular.
Drops a Food Ration.

Father Grigori wields a shotgun, but this time he also spawns Manhacks at a pretty fast rate.
This will definitely make the battle quite a bit harder.
Drops a Food Ration.

Father Grigori leaves The Ratman Room, and goes to the middle of grass field outside, and starts dropping bombs around himself.
Drops a Food Ration.

A Female_01 citizen NPC that will attempt to run from the Hangar into the large building near water.
Player has to block her while she attempts doing just that, which will grant you the comedic transformation of the woman into the Curse Detector.

The places where you can listen to these are usually marked with a round black spot, that resembles an explosion crater.

• Literally on the other side of Ratman's pool (after finishing which you can enter 2796-1646-20 in the console and you will be required to help Grigori fight off the fast zombies)
  
• On the wall inside the tunnels
  
• Down the stairs on the floor of the entrance to the white and mirror rooms
  
• Inside the Oracle Vent, though it just explodes and ignites the player
  
• Inside the Dark Room

There is also one on top of the building near the pool of water that is (I assume) required to progress, because the Vent explicitly tells you about it, and (I think but I'm not too sure) the props that are needed for the altar start to spawn after you activate it.

• The gm13_tier command always says that there's not enough power to set it above 1. Unlike previously thought, the number is not picked randomly on server creation, and always stays at 1.
  
• It also seems that at the date of writing this article, there is only one finished tier on this map. The second tier I assume is being worked on, but so far has one single event implemented. Third and Fourth don't even exist inside the scripts folder yet.
  
• Fun fact: GMod's 13 beta gm_construct really did have these weird creepy mine-shafts and tunnels, though I assume they chose not to keep them, as I do not remember the map having them when GMod 13 Public Beta was out.