To get the true ending:
- Restore vestiges of every Attendant of the Relics before fighting a successor. Vestiges are scattered throughout the map, but they are located in the same area of the attendant. After fighting the Successor you must return to the place where the attendant used to wait, this because they drop an Eos vestige. The Eos vestige is used to restore the vestiges of the last attendant located at the lift before you enter the last area of the game. I am not sure if you need to have Io as your companion, but I would suggest this.
- When you have restored all the vestiges of an attendant and you fight the successor, you'll have to interact with them in the memory instead of walking away (which will give you another ending).
To get the good ending:
- Make sure to save every successor (explained above), but do not restore the vestiges at the last attendant before the final boss.
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To get the neutral ending:
- You'll be required to atleast save one of the successors. You can then walk away from the remaining successors. To save a successor you MUST acquire all vestiges for the attendant located before the boss room.
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To get the bad ending:
- You will need to walk away from every successor inside the memory (after fighting the boss).
Code Vein lets you choose to either play NG+ with enhanced enemies or at the same level of your first run. If you want to speedrun this achievement, I recommend doing the latter. I'm doing my last run for the bad ending. Good hunting everyone!
Updated thanks to: HappyTheDevil