S.T.A.L.K.E.R.

"This is the most difficult and rare achievement in the Zone. It is awarded to only a few. Earning this achievement means that you:
- Successfully completed at least 99% of all the tasks in the game.
- Performed every task with due responsibility and thoroughness, while avoiding extremely inefficient and obviously ill-conceived methods of achieving the goal.
- Carefully studied all the dialogues and all the information that was provided during the playthrough.
- Found all the hard-to-reach and bonus stashes, as well as all the main stashes, which were not provided with clues, the so-called Easter Eggs.
- Did not miss an opportunity to help others, were sufficiently kind, noble, attentive to the friendly NPCs and not excessively greedy during your stay in the Zone.
- Have never cheated. At least, you never took advantage of those methods of cheating that were tracked during the playthrough.
In other words, this rare achievement shows that you have acquired the highest skill in the Zone andlearned almost all of its secrets. Therefore, one can say with certainty that you are a true S.T.A.L.K.E.R.!!!"
S.T.A.L.K.E.R. is the ultimate achievement in OP 2.2 that requires completing the vast majority of the content of the game - quests, stashes, secrets, optional and non-obvious stuff such as aiming for the best/hardest outcome in most missions.
This achievement is really hard to get, because forgetting or messing up even a minor thing will block it, and you will only discover that hundreds of hours later, at the end of the game. Most players who target it in their first run fail.
== Triggering ==
You have two attempts to get S.T.A.L.K.E.R., 100 seconds into the rolling credits after either the Bloody Endgame storyline or Video Archive (you have to complete both anyway). The check only succeeds if you have all requirements listed below.
== Optionals ==
A few notable quests and optionals are omitted from S.T.A.L.K.E.R.; they are covered by other major achievements:
* New Game - complete all chain quests.
* Card Player - find all 36 cards from Kuznetsov's deck.
* Zone Expert - find all Wesker's stashes.
* Vivid Nightmare - only play on Nightmare.
* No-Cheater - never use cheats built into the game options.
== Blockers ==
This game is multiple hundreds hours long, and you can easily miss or forget about something minor, blocking the achievement. However there are also things that you should not do.
Doing any of the following will block S.T.A.L.K.E.R. immediately:
* Failing any quest (must be listed as failed in the PDA, cyclic and chain quests don't count here).
* Failing 10 cyclic or chain quests (NPCs will also stop offer any cyclics in that case).
* Letting any quest givers die, even if you resurrect them with a Living Heart afterwards.
* Getting the Cheater achievement. It's given for sneaking into the areas you aren't allowed yet using some kind of bug (which is also very likely to break the game), and also for hacking your money with an external program.
* Having 30,000,000RU in your inventory at any single point of time. You'll get an achievement for that.
* Spawning more than 5 quest items with the spawner built into the options. You'll get another achievement for that.
* Bribing Sidorovich to make peace with either Kuznetsov or Sherstyuk.
* Bribing Sidorovich to make peace with any faction three times or more. You're forced to do this twice in the late game (it's part of the story), so essentially it's just one time and your achievement is blocked.
* Buying Voronin's documents from Informant.
* Drinking ~70 or more vodka bottles throughout the entire game, including the quest bottles (for example quests like Get RG-6 from "Freedom". You get Alcoholic achievement for that, which will let you drink one bottle more per day without dying, but this achievement will be blocked.
== Requirements ==
You need all of the following to earn S.T.A.L.K.E.R.:
=== Medical-related achievements: ===
* Guardian Angel: Save and escort all 5 surviving scientists in the Wild Territory during the Meet Kruglov mission, including Kruglov himself.
* Rescue Service:
** Save all Duty members held captive by the bandits in the Dark Valley. See Rescue "Duty" member for details; Bullet, Max Lyuber, Demyan Ivanhoe and Sergey the Shaggy must stay alive and make it to the Dump.
** Attempting to save the Psycho in the Army Warehouses by wounding him and giving him a medkit. He will still try to kill you, but you at least tried to save him. See Army Warehouses#Crazy stalker.
* Doctor Dolittle: Heal 20 wounded stalkers using medkits. It's difficult to do, but the game is huge and provides you with enough runway for this, as long as you are trying to help every wounded stalker you see on every occasion. It can also be cheesed by healing the crazy stalker multiple times or using the OP 2.2 way and injure the neutral stalkers by detonating gas cans nearby.
* Mother Teresa: Revive Chemist by giving him a Living Heart artifact in the Forgotten Forest, during the Inferno quest.
* Pharmacist: Bribe Garik 30 times using antirads or first aid kits (2 first aid kits, 1 scientific or military first aid).
* Keeper of Life: Never let a single "story" NPC die (the ones that give a beeping message if they are killed). You will be blocked from getting this achievement even if you resurrect the dead NPC with a Living Heart, so you need to reload if anyone important dies.
** This achievement is rewarded at the end of the Sniper plotline.
=== Plotline-related achievements: ===
* Task Performer: Never fail a single mission from the main plot or side quests. Note: this achievement doesn't count cyclic quest failures - you could fail 9 cyclics without blocking S.T.A.L.K.E.R.
** This achievement is rewarded after speaking to Natasha after completing the Hearth and home questline.
* Chef: Complete the Sniper plotline by cooking Wulf embryo for Fenrir. The recipe is very complex and some parts are gated behind certain side quests unrelated to Sniper:
** Assassination of the Shadow of the Monolith and the subsequent Hunter's stash
** Diamond skulls
** The monsters' parts (Artifact Klondike)
** Mole's PDA (either peaceful or aggressive option)
** Walking dead
** entire Invisible stashes questline
* Cartographer: Solve the mystery of the Floating Islands, specifically make it to the final island in the Hospital.
* Gromov: Complete the Volazar questline by getting all modules for the Modified Nanosuit, specifically you will get the achievement from Assembling the modules: Part 10.
* Defender of the Zone: Complete the Wave of Mutants plotline (you will get the achievement from The Wave: Last Battle)
* Librarian: Collect all 416 documents from Voronin's infamous document scavenger hunt. You will get the achievement in Just Reward, but only if you didn't buy any documents from Informer!
* Collector: Find every single Collector's stash. There are 53 stashes total; you will get the achievement when speaking to Sidorovich after finding the final stash at Beauty and the Beast.
* Sniper God: Complete four missions related to long range sniping:
** Liberation of the Slavik the Bullfinch's squad during the Floating Islands questline
** Sharpshooter from Hog (shoot 130 bottles out of 135)
** Crumb the deal from Glimmer Man (intercept the military and ecologist trade in Jupiter)
** Test of Strength from Captain Ivanstov (Help a Duty recruit named Viking fight against zombies from afar)
* Sponsor: Complete all of Sidorovich's contributions for a grand total of 5.5 million RU.
* Liberator: Complete the Valley of Whispers questline.
* Paparazzi: Complete most missions that involve taking pictures (except for The Mad Doctor which is not counted here) 
** Photo of an embryo
** Wildlife photography
** Anomaly photos
** Commemorative picture
** Low-key hunting
** Wolf's lair
** Gauss installation
** Deja Vu
** NS storyline missions which don't use Missing photographer's camera:
*** Picture of the Monolith
*** Picture for Klyonov
*** Panther's murderer
*** Intruder
* Detective: Complete Strelok's and Kostya's stash-related questlines while picking up all bonus stashes. More exactly, you need to loot:
** Strelok's last stash in the sky over Radar
** Strelok's bonus stash given after that (the one with Groza "Sniper" in Dark Valley, see the same page)
** The last one of Old-timer's backup stashes, containing the random teleporter device.
* Finish the Video archive by collecting all discs.
* Chesser: Complete the Chess Player plotlines. Recluse must be eliminated in Arguments and Facts and Murzo in Bloody endgame.
** This achievement is rewarded after speaking to Natasha at the end of the Hearth and home questline, making that a third requirement.
=== Item-related achievements ===
* Cognizant of the Zone: Collect all manual teleporters available in the game (i.e., the ones that require the teleport artifact). Refer to Fast travel#Manual teleport for the full list. The teleports are gated behind the following quests and secrets:
** Bonus stash: Yantar, accessible at any time
** Strelok's PDA, the teleport is on a concrete slab near Wish Granter and can be hard to see
** "Gagarin" prototype storyline
** Hostel storyline
** Interpol in the Zone storyline
** Certain NPC chain quests. Note: many items in these are gated behind other quests!
** Some quests which are directly required for S.T.A.L.K.E.R. anyway:
*** Sniper storyline, first Fenrir's quests
*** Stash at the Military Laboratory (first quest in the Volazar's stash series)
*** Strelok's stashes
*** Mystery of the Islands storyline, in the bonus stashes for most islands
* Gun Baron: Collect all unique guns available in the game. It's enough to "touch" them (i.e. have them in your inventory at least once, independently of each other). You will need to get a specific list of guns for that achievement. Most of them are gated behind certain quests, and some guns are easily missable in particular:
** 6G30 "Zeus" from Insurance - can be missed if not grabbed in time
** McMillan CS5 "7.92x57" - only available in the [ stash in Chernobyl-1] or in Sidorovich's Contributions
** Strikeback from Wildlife photography or Fiend's chain 
** Honorary Vintorez from Special magazine
** Hostel from Hostel storyline
** VSSK "Backwoods" from either Sniper hunting (broken) or Hard target (new)
** KORD "Tank" Machine Gun from either Sidorovich's Contributions or Lootoholic's chain
** HK 417 "7.92" from either Disguise or The mercenary bar's safe in Chernobyl-1
** FN F2000 "Paratrooper" from Kuznetsov's deck (blueprints)
** FN F2000 "Executioner" from either paying 250000 RU from Akim or Lukash's chain
** KS-23 from Alexandrych's Jupiter stash
** Ghost's Gauss from Trap for Beast
=== Miscellaneous achievements ===
* Hero of the Zone: Achieve 30,000 points on the leaderboard.