Rescue "Duty" member

When you walk into the Dark Valley the first time and talk to Bullet in Liberate the "Duty" member, he will mention that two stalkers from his squad, Sergey the Shaggy and Demyan Ivanhoe, are still being held captive by the bandits. You can try to save them.
== Walkthrough ==
=== The easy way ===
This is the canonical OP-2.2 way to do it, which is simple but needs 40000RU.
* Enter the Dark Valley from Dump. Complete Liberate the "Duty" member, healing Mongol in the process and making the bandits neutral.
* Talk to Hog and pay 40000RU as ransom for the captives.
* Free Shaggy. Heal Ivanhoe. Escort them to Dump, ensuring that they actually walked into the transition.
* Go to the Duty checkpoint at Dump and verify that all four squad members are alive and sit around the campfire. Talk to Ivanhoe and get your Living Heart artifact as a reward.
Note that this way is not available after Find the documents in X-18 (after giving the docs to Barkeep). Also, the door to Shaggy's cell must stay closed and both prisoners must be alive for the buyout to be an option. 
=== The hard way ===
This is the obscure, long, and painful way (a.k.a. the real OP way), in which you don't have to pay any money, and earn some scrapes and bruises and also stalker rank in the process (you will probably need it for Arena).
* Accept Inferno from Sidorovich and Find the documents in X-18 from Barkeep. This way you will get the transition from Forgotten Forest to Dark Valley.
* Instead of entering through the Dump, enter the Dark Valley from the Forgotten Forest via the newly obtained transition. While in the Forgotten Forest, keep in mind that talking to Fistful in Inferno temporarily disables the transition to Dark Valley, so ignore that quest for now.
* After entering the Dark Valley, don't approach Bullet yet.
* Save Max Lyuber by killing his guards. Sneak up to them from the east or snipe them from the west, as they kill Max too fast if they spot you when approaching by the road. Max will sprint towards the bus and stop, unable to talk at this time.
* A bloodsucker in the [ northeast forest] can mess up the process, so check in advance if it spawned there and kill it if so.
* Wipe the entire bandit base out, or at least those who stand in the Shaggy's path to escape.
** Don't heal Ivanhoe and don't free Shaggy yet.
** Keep your eye on the minimap for two yellow dots; the Dutyers must not die!
** There's actually very few bandits inside the base at first. Crossing the red line on the picture or approaching Hog's building will spawn a lot more, though. They will likely kill Shaggy. To avoid the massive bandit spawn, you can climb out of the window on the first floor and enter the basement directly. 
** Don't kill Hog and Skimpy, they are quest givers. They won't shoot anyway, and will speak even if hostile, similar to Kuznetsov and some others. No need to kill anyone who's not standing in the way, either (Hog's guards, for example).
* Now when the path to escape is clear, heal Ivanhoe and free Shaggy. Sell them a broken shotgun from the bandits, and a magazine worth of ammo, so they don't scream and panic erratically while escaping. If you want, you can trigger the spawn and have fun wiping the rest of the base; just make sure both Dutyers left the potential combat zone.
* Travel to Bullet (near the Dump entrance), and launch the Liberate the "Duty" member quest by touching the transition and selecting "No".
* Listen to Bullet and Mongol talking, the quest will start and immediately finish (as you already saved Max Lyuber). Sometimes Mongol kicks out the gun from Bullet's hands after being helped, or outright kills him with one punch. Reload if that happens.
* Heal Mongol (which also counts for the purposes of The rescue), making the Dark Valley bandits neutral.
* Speak to Bullet, receive your reward. Speak to Max, get the quest to rescue the rest of the squad.
* Escort other squad members, and ensure that they have left the location to Dump.
* Go to the Duty checkpoint at Dump and verify that all four squad members are alive and sit around the campfire. Talk to Ivanhoe and get your Living Heart artifact as a reward.
Here's the video on freeing the squad the "hard" way. It doesn't show the trick with avoiding the massive spawn, though.
== Notes ==
* You can only pay ransom until you enter X-18. After that, this option will be unavailable.
* Neither Shaggy nor Ivanhoe are essential for the rest of the game, and they don't give a red beeping message when dying. However they also have cyclic and chain quests, hence it blocks the New Game achievement, and without them you won't get the Guardian Angel achievement, which blocks the S.T.A.L.K.E.R..
* If you refuse to free them, they will be killed by bandits, so if you're going to do it, do it during your first visit to the Dark Valley.
* This quest is the one of the three places go get the Living Heart artifact in the early game, with the other two being Meet Kruglov/Escort Kruglov to Yantar, and Army Warehouses#Crazy stalker.
* After healing Mongol, all bandits in Dark Valley will become neutral to you for the rest of the game. This neutrality is also easy to lose if you kill too much of them. Mongol is not needed for anything anymore, so there's no point in protecting him.
* Technically, you could just shoot Mongol at the spot without healing him, as Hog and Skimpy will talk even if hostile. The only difference would be the necessity to wipe the base out every time you want to talk with them. There will be two quests down the line that require you to speak with ordinary bandits, but they all are likely beatable with the them being hostile as well (if you decide to do that, make sure to report in the wiki as nobody actually verified that in the game).
== Bugs ==
* This mission is notoriously glitchy, which is the vanilla SoC legacy. Dutyers can panic or walk back to the start of the route occasionally, be killed in offline, all sorts of shenanigans can happen. Sell them one of the broken shotguns picked up from the bandits, and make sure they actually left the location.
* Mongol is hostile to the squad even if you paid the ransom. Watch out for him picking anybody's gun. You can blow him up with a canister if needed.
* Healing Messer in the gas station before completing this quest usually has him charging into the bandit base and killing Ivanhoe.
* After saving Max, Bullet will offer you the quest Ammunition delivery, but only once. If you refuse or close the dialogue without accepting it, he won't talk about it ever again. The way the script is written, this probably wasn't intended by the devs.