This guide details the requirements to unlock the Ark Angel achievement for which you need to prevent every avoidable descendant death across all episodes of The Descendant. This guide ensures you have the highest surviving descendants count possible while also leading to the best ending.

As these actions are spread across all episodes and include spoilers, every episode is its own section and I've tried making the descriptions actionable without disclosing too much but sometimes spoilers aren't avoidable. Therefore, beware of spoilers given the nature of the actions involved.

This guide has general puzzle solutions but not necessarily extremely detailed ones so you still have some fun figuring things out and also because the game can be a chore to replay given it has unskippable dialogue and animations so detailing exact colours, etc would be tedious. Since the puzzles are simple, the provided solutions should be sufficient to get you through (should you need them).

You need to fix the water heater in time (less than 10 minutes). It's easy to do but you need to be prompt as the timer keeps running while the unskippable descriptions, animations and dialogue are playing.

1. After you enter the room with a downward slope, run (double click to the point you want to go) to the door after the corridor with a fallen shelves and an interactable list of floors and enter the room.
   
2. On the right hand side of this room is a piece of paper facing an interactable switched off computer monitor with the code to the door. View the code and note it down.
   
3. On the far end of the room from your entry point is a locked door with a keypad. Enter the code from the previous step and go through.
   
4. Run down the corridor and enter the next room
   
5. Run to the elevator, enter it and press on the panel with the buttons
   
6. There will be a power cut and you'll need to fix the panel by connecting the two long wires. This triggers one of the unavoidable descendant deaths in the game.
   
7. Run down the corridor and towards the wall of steam and examine it. The game will tell you Mia cannot pass.
   
8. Run back to the valves and examine them. The game informs you of the missing valve.
   
9. Run back to the beginning of the room and look to the right where there are some yellow cylinders to find a valve.
   
10. Run back to the valves, install the missing valve and solve the puzzle by looking at the poster on the top right and tracing back the pipes and turning the valves in the order defined there.
    
11. Run to the door after the now removed steam wall, enter the boiler room.
    
12. Run to the boiler, switch off the boiler and note the order of the pipes as you remove them and examine the seal.
    
13. Run towards the ladder and look at the cardboard box to its left to find a replacement seal
    
14. Run back to the boiler, install the new seal which will drop to a lower level.
    
15. Run to the ladder, go down to the lower level
    
16. Pass through the pipes, turn the valve, pass through the next set of pipes and pick up the seal.
    
17. Repeat the previous steps in reverse and go back up the ladder
    
18. Run towards the boiler and replace the seal, put back the pipes in order and switch the boiler back on.

Note: If you complete the above before Mia starts complaining about it getting too hot and automatically leaving the boiler room, you will also unlock the "The Fast and The Studious" achievement. I encountered a case where while she's in the lower level she said that but didn't leave the room and I was still able to successfully fix the issue without getting the achievement. As that might be a bug, don't count on it for your own playthrough.

1 - When fixing the elevator there is a short power cut that causes one unavoidable descendant death.

Note: Prior to the end of the episode, solving the startup sequence puzzle entirely without mistakes (thereby unlocking the "Great Kid, Don’t Get Cocky" achievement) may be required but I have not tested the case where that puzzle is solved with mistakes so cannot confirm. In case you need the solution for that puzzle, here are the options as defined in the Achievement guide by "kissa <3" under the "Well done, man, but don’t worry" title:

1. Coolant - 1-3-2
   
2. Plumbing - back flow
   
3. Wires - Detach all black and blue wires
   
4. Grounding - Connect it to socket G86

At the end of the episode, after Mia fixes the tower and a choice comes up, choose to finish the job instead of attempting to run away.

None

When Donnie has the option to unlock more options at the computer, don't and choose to leave it alone.

Note: You may want to copy the save before this so you can explore the interesting information revealed when you do try unlocking options. As this causes avoidable descendant deaths, make sure to load your primary save again and not unlocking any options in it before continuing to play.

None

At the beginning of the chapter, Mia must activate the sprinklers by repairing the valve to put out the fire. The missing part is found in a toolbox after going forward then left after examining the sprinkler.

Note: If after examining the sprinkler you go forward then right, you can go back to the toolbox and repair the sprinkler. However, if after going right you choose to go between the boxes instead if going back you will have failed this requirement as you can only exit the room and thus cause avoidable descendant deaths. Reload the save if and replay that room to remedy the situation.

7 - Due to Silas' actions revealed in this episode

Near the end of the chapter, when presented with the choice to shoot someone, don't.

None (the one in the flashback sequence doesn't count as it is in another Ark).

Upon completing episode 5, you should have 100 descendants left and the Ark Angel achievement will unlock while also giving you the best ending.

Note: A comment by LvlLord on 29 Dec, 2021 to the Achievement guide by "kissa <3" states that this achievement can also be unlocked with 95 descendants left after hacking the computer twice in episode 2 and shooting the senator but I did not test that myself so cannot confirm it. Shooting the senator locks you out of the best ending as the door doesn't unlock at the end (although that might be implied). This guide presents the highest surviving descendants count possible while also leading to the best ending so this information is mentioned here for reference only.