How to Acquire
To unlock this achievement, Billie Lurk must choose to free the the Outsider from the shackles of the Void instead of killing him during the mission A Hole in the World. This achievement can be earned only after reading Malchiodi's Diary and learning the truth about the Outsider's Mark.
After interacting with the Eye of the Dead God, turn around and pilfer Malchiodi's Key and Malchiodi's Last Notes from the corpse attached to the wall. With the key now in Billie's possession, backtrack to the locked room with "entry forbidden" painted above its door and unlock it. The code to Malchiodi's chest can be deciphered by isolating the three characters that make the Outsider's Mark and shifting the corresponding digits until the right combination is found. Once the chest is unlocked, read the entry in Malchiodi's diary and proceed to the Ritual Hold. After speaking to Daud for the first time, approach him again and convince him to free the Outsider by choosing the appropriate dialogue options or by continuing to talk to him. The achievement unlocks after the unskippable ending cinematic.