So, for the Soulless Ending, all you need to do is:
1: At the endless hallway, just keep walking upwards until unknown jumpscares you. Your game should crash.
2. When you reopen the game, you'll be met by the unknown, she will chat with you for a bit. Ask you to rub her belly. You just need to do what she asks.
3. She will ask you for your soul, say yes.
4. Finish the game normally, then create a new game.
If it is done right, unknown will interfere with the story at the start of the game.
The game gets stuck at unknown after it, but you only need to delete the arquive "firstrun", on the game folder to return your game to normal.