The Dark Valley is the location of the X-18 lab, and has scattered radiation pockets and anomalies.
One or more game breaking glitches occur in this section. As such, you should have a saved game before entering the section.
Free the Dutyer
When you first enter the Dark Valley, you will see two members of Duty interrogating a downed bandit, before they recruit you for a quick mission. They will move to an ambush point at an intersection, and instruct you to take cover. The captive will pass first, and the two bandits lag behind him by some distance. Rescuing the prisoner is easy if you follow the instructions, since the enemy is flanked and separated from the hostage.
The bandit base is located at the north of the Dark Valley. There are two tasks here - the first is to optionally rescue a Dutyer from the prison, next is to kill the bandit leader Borov for the key to X-18. In the building to the north-east, there is also an injured stalker that you can help by giving a medikit (who will then tell you about a gauss rifle for "sale" for 800 RU.)
There are two major entrances to the bandit base. The front entrance is suitable for those liking a quick frontal assault, but it can put the prisoner at slightly more risk. The sewer entrance is slower, but allows you to get a bit closer and in position to catch the bandits off-guard, and also gives you opportunity to avoid some of the bandits.
Side quest: Free the Dutyer from prison
This is an optional side quest, which provides a small reputation boost to the Duty faction.
There is a Dutyer imprisoned in the bandit base. To reach him, you first need to raid the strongroom, and head up the stairs in the north-east corner. Continue through the sole path until you reach the spiral staircase branch. Head downward to the lowest floor, and turn left. This route will lead to the prisoner, where you can open the door by pressing the switch on the wall.
If you can't open the door, you can recover from this by exiting the map and re-entering.
Find the documents in the underground lab
To enter Lab X-18, you need a key from the Bandit Base. Enter the base either through the sewer or the front entrance, and proceed into the main building.
At the spiral staircase, continue forward either on the current level or one floor down. The bandits ahead will mostly attack from behind cover and adjacent rooms. When you reach the atrium, there are also bandits waiting in the near corners of the atrium, ready to attack when you peek in.
You will need to find someone named Borov, and kill him. He is located on the second floor, in the room on the opposite side of the atrium. When he is killed, search the body to (silently) retrieve his key. You should receive the objective to leave the base - you may either backtrack to the entrance you use, or follow the waypoint to take the bridge back to the main road.
Note: The game may not allow you to open the door to the base. If this is the case, try speaking to Barkeep again and searching Borov's corpse. If the door is still locked, you may need to reload a previous save to progress hte main story (although the game isn't technically unwinnable.)
The X-18 installation is described on the following page, Lab X-18.
Get out of the Dark Valley
When you leave Lab X-18, you receive the message that the route back to Garbage is blocked off. You will need to take the south exit.
On the way south, you will see an abandoned pig farm. Right now, the only significant interaction is speaking with one of the loners in the front, who offers a gauss gun for only 800 RU only to swindle you right away. Avoid them until you obtain the quest to kill the leader of the group.
After heading south, you will return to Cordon, arriving from a blocked-off tunnel (where you would have found the perfected suit). There are additional tasks back in Cordon as well.