To unlock this achievement and finish Book 3 you must get Kian onto the giant cloudship.
After bringing Zoe to Arcadia and completing her short sequence you will be back in control of Kian at Rebel HQ.
Head downstairs through the door on the left and into the boat.
You hear two guards talking. Wait for one to move and head to the stationary guard. When he turns to pee take him down and pull the lever.
After a cutscene head into the arch and through the door on the right (in the middle of arch). Sneak up on the next guard and attack.
You now need to ring the bell.
once you ring the bell there is a QTE where you must quickly decided whether or not to take Likho. This decision affects future achievements.
Whether you take Likho or not the cutscene will play and the achievement unlocks