This achievement is tied to 1 of the 3 possible endings.
To get this ending you only have to do 2 things:
1.
*** Spoiler - click to reveal ***When given the choice to either give Sophia peace or to let her live, choose to let her live. This choice will happen near the end of the game shortly after killing Laxasia The Complete.
2.
It’s also worth noting that in order to be able to do #2 you must have acquired enough humanity in order to get the refuse response to show up. The exact amount needed is unknown, but I know it doesn’t require much. As long as you listen to records all the way through and lie a few times you should be set.
If you have any questions, feel free to ask!
Credit to WhyattThrash for the information.