999 contains 6 completely different endings. 5 of them are not missable and they reward you with an achievement. The Coffin Ending however is missable (but there`s no achievement for it either). Below are the descriptions how to reach the different endings.
There are no missable achievements!





















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Ax Ending

Flowchart Paths: Left, Left, Left or Right, Left, Left or Right, Middle, Left or Left, Middle, Left
Door-Decisions: 5, 7, 1 or 5, 8, 1 or 4, 8, 1
Puzzle Rooms crossed: chart room, captains quarters

Saw the Ax Ending in 999
571, 581, 481







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Zero Lost Ending

Flowchart Paths: Right, Middle, Middle
Door-Decisions: 5, 8, 6
Puzzle Rooms crossed: 1st class cabin, casino, laboratory, engine room, cargo room

Saw the Zero Lost Ending in 999
586 (+ all the red keys on the flowchart)






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Knife Ending

Flowchart Paths: Left, Middle, Middle or Right, Left, Middle or Left, Left, Middle
Door-Decisions: 4, 8, 6 or 5, 7, 6 or 4, 7, 6
Puzzle Rooms crossed: engine room, cargo room

Saw the Knife Ending in 999
486, 576, 476






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Submarine Ending

Flowchart Paths: choose once a path to the right (except at the beginning)
Door-Decisions: 4, 3, 2 or 5, 3, 2 or 4, 8, 2 or 4, 7, 2 or 5, 8, 2 or 5, 7, 2
Puzzle Rooms crossed: prison, torture chamber

Saw the submarine Ending in 999
432, 532, 482, 472, 582, 572






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True Ending (Coffin Ending)
you need to have reached the Zero Lost Ending at least once before to reach this ending. If you haven`t finished the Zero Lost Ending, you will get the coffin ending in this path.

Flowchart Paths: Left, Left, Left
Door-Decisions: 4, 7, 1
Puzzle Rooms crossed: 2nd class apartment, kitchen, operation room, chart room, captains quarters
Requirements:
Meet all of the white key events: You have to reach all of the following requirements to gain this ending:
1. Accept the bookmark from Santa (2nd class cabin) 🔑
2. Talk with June in the freezer about the ice-9 (kitchen) 🔑
3. Bring up the conversation about ice-9 with Seven (operation room) 🔑
4. Give Clover the bookmark you got from Santa (operation room) 🔑
5. Complete the Zero Lost Ending at least once! 🔑

If you already completed the Zero Lost Ending once, you won`t have any chance to reach the Coffin Ending anymore other than deleting your save data. But don`t worry. This ending is not really an ending and does not reward you with an achievement. It is therefore not necessary.


Reach the coffin ending in 999
471



Reach the true ending in 999 🔒
471

You should have unlocked every of these achievements if you unlock all endings, but we`ll make sure. These are the rest of the achievements

A sixth sense of what`s coming
Saw all previews in 999


There are 2 previews that you are able to watch:
(a) unlocked by Zero Lost Ending
(b) unlocked by Cliffhanger ending or other endings




Seek a way out again
Gained all memories of escape in 999


You`ll have to complete every escape-the-room puzzle. This should be clear, just look what you missed at your flowchart:

Door 4: 2nd class cabin, kitchen
Door 5: 1st class apartment, casino
Door 7: operation room
Door 8: laboratory
Door 3: shower room
Door 1: charter room, Captains quarters
Door 6: engine room, cargo room
Door 2: prison, torture chamber




Don`t forget your towel
Finished all scenarios in 999


This one should unlock automatically if you reached all previously described endings and if you gained all memories of escape.



Hope my guide helped ;)