Conqueror of the Abyss
Complete the game on Hard difficulty.

• Self-explanatory.

Armed and Ready
Recover the arming code for the Soviet nuclear bomb.

• Before the last mission, there will be an optional mission to get arming codes. Just finish the mission to unlock this achievement.

You can see the mission at the map here:

The Nuclear Option
Detonate the Soviet nuclear bomb.

• Getting the arming codes is required. Then you have to arm the bomb and survive for a bit. Or you can try to do the other ending achievement and save before killing the boss and arm the bomb, then kill the boss.

Home Sweet Home
Defeat the Captain Ascendant.

• Just kill the boss without arming the bomb.

You just have to kill some enemies and inspect them. I don't know if some of these are missable.

Strange Beasts
Inspect a dead Scyphozoa.

• This will be the first enemy you will encounter at the campaign. Just go near it right click its body after killing it.

Lurking Menace
Inspect a dead Spineskulker.

• Self-explanatory.

Under Pressure
Inspect a dead Burst Pod.

• Self-explanatory.

Warped Fauna
Inspect a dead Infected Pod.

• Self-explanatory.

Predator
Inspect a dead Murkfiend.

• Self-explanatory.

Vessel of Corruption
Inspect a dead Vessel.

• Self-explanatory. These ones will appear more when corruption is higher.

The Parasite
Inspect a dead Voidspawn.

• Self-explanatory. These creatures spawn from corruption.

The Priesthood
Inspect a dead Fathom Seer.

• Self-explanatory.

Hunters of the Trench
Inspect a dead Stalking Tentacle.

• Self-explanatory. You will see these enemies near the end.

Not So Cold War
Inspect a dead Soviet Diver.

• Self-explanatory. You will see these enemies near the end.

The Harvest Begins
Inspect a dead Harvester.

• Self-explanatory.

The Insatiable
Defeat the First to Hunger.

• First boss, you can't miss it and you do not have inspect it.

Beyond Reality
Defeat the Gatekeeper.

• Second boss, you can't miss it and you do not have to inspect it.

Still Counts
Have an enemy kill another enemy.

• You can use charm or mark skill to make enemies to attack each other or you can just use a vessel or pod explosion.

Barrage
Deal 250 damage to enemies in a single turn.

• You can unlock this achievement easily as you progress.

Massacre
Kill five or more enemies in a single turn.

• I don't know how I got this achievement. I think vessels triggered each other. Using vessels is not a bad idea for this achievement.

Never Liked Them Anyway
Kill one of your divers with friendly fire.

• A miss with speargun, AOE damage or madness can do the job. It goes without saying but save before trying for this achievement.

Transformation
Mutate one of your divers.

• Pretty simple one. Just use a void marrow to mutate one of your crew.

Superhuman
Have a single diver undergo four Mutations.

• You have to use 8 void marrows or less with the level 6 skill of mutation which can be unlocked for some divers (minimum 5).

Even Death May Die
Use the Profane Chapel to resurrect a dead diver.

• You need to build a Profane Chapel to an empty room and use one void marrow to resurrect the dead diver. You may want to save and load.

Good As New
Pump every section of the Salem.

• You just have to pump all of the water; you do not have to build or repair. So, it won't be problem.

Geared to the Teeth
Unlock one of the top-level nodes on the Mechanical track of the Enigma Board.

• Self-explanatory.

Survival of the Fittest
Unlock one of the top-level nodes on the Biological track of the Enigma Board.

• Self-explanatory.

The Dark Arts
Unlock one of the top-level nodes on the Eldritch track of the Enigma Board.

• Self-explanatory.

Combat Veteran
Kill 500 enemies.

• Self-explanatory. I got this one towards the end of the campaign.

Unveiled Mysteries
Uncover 1,000 Clues.

• You can get this achievement near the end. If not, you will get it while playing endless mode.

Legendary Blow
Deal 150 damage with one critical attack.

• This one was hard for me. I wasn't able to do it while I'm playing campaign (hard difficulty). Then, I tried an easy endless run and opened the speargun damage upgrades (mechanical tree) and critical damage upgrade (biological tree). I used a special speargun from Soviet Cache missions, another special item which guarantees critical strike from Soviet Cache missions, spotter skill and dart skill which decreases the armour. First, I used the dart, then spotter skill and critical item, lastly special speargun. My choice of enemy was an infectious pod.

Endless Master
Earn 20,000 score on Endless Mode.

• Hard gives the most points but easy is much more secure. It isn't that much of a time-consuming achievement.

Thank you for checking my guide. If you have any suggestions or if you see any mistakes (information, typo, etc.), please let me know. Have a nice day!