Complete all 20 Conscience Dialogues
Conscience Dialogues are the white/black aura choices you get throughout the story. This achievement says “all 20”, but there are far more than 20 opportunities to answer the conscience dialogues, and it seems to count them in an unknown way unrelated to what the game actually saves under the Journal > Dialogues > Conscience section of the Map menu. You will have many opportunities through the game during story and sidequests for these dialogues, so just keep playing and you will earn this eventually.