This is a story-related achievement and cannot be missed. It is the
only sequence-related achievement in the DLC.
*** Spoiler - click to reveal ***This sequence takes place at Lambeth Asylum, which is a location you visited during the main game. You have two optional objectives (paraphrasing):
- Enter the Asylum without being detected
- Kill Jack the Ripper's three henchmen
You have a narrow area for entrance into the asylum grounds, and you can either use Fear Bombs to make your way up to the asylum, or you can climb the walls, then the trees and navigate that way (there is a chest to the left of the grounds entrance, and Helix Glitches to the right of the grounds entrance up a cart ramp after you enter the gate, and another at the top of a spire at the rear of the asylum. These are the only collectibles outside.
Once you have entered, you will not only have to deal with the three henchmen mentioned above, but you will also have to deal with the inmates Jack let loose at the end of the previous mission. The three henchmen are located in different sections of the asylum, so you can take your time taking them out. When you've taken out the henchmen and are ready to proceed, make your way to the auditorium in the center, where Jack has left a message for Evie that the "final surprise" is waiting for her down in the basement. Once you reach the basement (there is a chest down here along the way), there will be a short cutscene where Evie may have noticed Jacob in a cell, another another with Jack before the final boss fight starts.
The basement is a small area with two levels. The top level is wooden floors that are around the outside of the room, with one small gap you will have to jump across. There are spots you can hide/take cover up here. The lower level is dirt, and there are stairs leading up to the top level here, as well as a makeshift ladder. In two opposite corners are winding staircases you can use to gain your advantages.
The boss fight has three basic phases to it, and for each you will have to either attack him from above or sneak up on him from behind. Once you get close enough to fight Jack, you will spam the button just as you have throughout the DLC for the Brutal Takedowns. You will have to do this for the first two phases, and you have to sneak - otherwise, Jack will see you and frontal attacks do not damage him. Once you have successfully made the first two attacks, he will throw a Fear Bomb and the final phase begins. The doors on the top level will begin to allow additional inmates to come into the basement, and you'll have to deal with them as well as Jack (as far as I could tell, these are infinite spawns, so you can't stay in one place too long even if you choose to fight them and you can't take all of them out just to get the fight back to just you and Jack). This phase becomes a little more phrenetic than the first two because of this, but the goal is the same - sneak attack from behind or above. Once you start the final attack, you will have to use the same Brutal Takedown tactics that Jack did in his sequences - spam the button and move the in the direction prompted for. You'll have to do this three times in a flurry to finally kill him.
Once the fight is over, you will get a final cutscene. Evie goes back to the cell Jacob was indeed in, and she dumps Jack's dead body on the floor as she enters. As she begins to comfort Jacob, Abberline comes into the cell. She begs Abberline to not let the world know that Jack was an Assassin.
The credits start here, and the achievement pops afterwards (you can hold X to fast-forward through them).