The Boyle Sisters are wealthy aristocrats in Dunwall. Lady Boyle is the financial backer of Hiram Burrows and therefore ensures his power over the Dunwall City Watch. To decrease Burrows' influence and money, the Loyalists send Corvo Attano to eliminate Lady Boyle. In each playthrough it is randomized who of the three sisters will be Burrows' mistress.
The three sisters
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The Loyalist Conspiracy is made aware of the Regent's mistress after the elimination of the Pendleton twins. To find out the identity of the mistress Corvo kidnaps Royal Physicist Anton Sokolov, who once painted her. However, while Sokolov reveals that the mistress is Lady Boyle he does not know which Boyle sister it is as he painted her from behind.
The Boyles are known for their vast fortune and lavish lifestyle. Party's hosted by the Boyles are highly anticipated events for the Dunwall aristocracy. It is during one of these parties Corvo infiltrates the Boyle manor to eliminate Lady Boyle. As the party is a costume party Corvo is able to walk freely among the guests once inside the manor's walls. During the party, the guests are invited to participate in a game: All three Boyle sisters wear the same costume but in different colors and the guests have to decide who sister wears which color. The guest who guesses correctly wins a Boyle heirloom, which Corvo can find and steal on the mansions attic.
During the party the Boyles display their fortune, allowing Corvo to steal some of it. Partygoers will comment, telling Corvo to stop, or sometimes encouraging him (a guest in the smoking room claims that "everyone does it" and says that a servant of his "sewed [him] an extra pocket, just for the occasion"). Being seen pickpocketing twice by the same guard within a space of about twenty seconds causes Corvo to become detected, so he must be careful not to pickpocket too often in the same area.
Finding out Lady Boyle's identity
To successfully complete the mission Corvo must find which sister is the Lord Regent's mistress and what color she is wearing. Corvo can also choose to kill all three sisters, therefor it is not needed to find out Lady Boyle’s identity. To find out Lady Boyle’s identity Corvo can bribe party-goer Miss White with a drink, and she will tell him two of the sisters' identities and the colors of their costumes. Her hints will not reveal which Lady Boyle is the target, however. This Corvo can find out by talking to Lord Brisby, who will tell Corvo the name of his target. Brisby also reveals that he is in love with Lady Boyle and begs Corvo to render her unconscious and give her to Brisby. Brisby promises that should Corvo do so Lady Boyle will never be heard of again.
Corvo can also search for clues in the second floor of the mansion which is closed off to party-goers. Each Boyle sister's room will have her diary open to the latest entry for Corvo to read. There will be a love letter from Hiram Burrows next to the diary of the "real" Lady Boyle, revealing her identity and what color costume she is wearing. A skeleton key to Dunwall Tower can also be found in the target's room which Corvo can use later while infiltrating Dunwall Tower.
If the name of the target is identified, it is easy to determine which color she is wearing, as the Heart will identify each sister by name when used.
If Corvo manages to kill all three Lady Boyles – stealthily or openly – he will succeed without having to find out which one is the right target, as it will be recognized as a "process of elimination."
Corvo can kill Lady Boyle with each weapon and skill at his disposal. Although Lady Boyle can be slain wherever she stands it is also possible to eliminate her away from the party, the location depending on which of the sister is Lady Boyle.
- If Esma is the target, Corvo preys on her insatiable sexual appetites. After some small talk, Esma will lead Corvo to her room. Under most circumstances, Corvo will be attacked if detected while on the second floor. However, if he is invited upstairs by Esma, the guards will not consider him hostile, allowing him to follow her to her room. Alternatively, telling Esma that Corvo is there to kill her leads her to believe it's all a sexual role-playing game. She can then be convinced to take him upstairs or meet him in the cellar.
- If Lydia is the target, Corvo preys on her fondness for music. Lydia will bring Corvo to the music room where she will ask him to impress her by playing something on the piano. As the music room is open to guests and servants, assassinating Lydia in this location is perhaps the most difficult and can lead to unwanted attention. Telling Lydia that Corvo is there to save her life causes her to distrust him, and she calls for the guards.
- If Waverly is the target, Corvo preys on her paranoia, saying that her life is in danger and he knows who is after her. Waverly says she will meet Corvo in the cellar where they can discuss the matter further. If Corvo asks to have a private conversation with her, Waverly will brusquely turn Corvo down. Telling Waverly that Corvo is there to assassinate her causes her to call for the guards.
As with all other assassination targets, if Corvo decides to murder Lady Boyle with his sword, a special animation plays. In it he will embrace her, then stab her in the stomach and gently place her corpse on the ground. If she is approached from behind, he will stab her in the shoulder, then as she falls to the ground he will stab her again in the neck.
If Corvo choses to dispose of Lady Boyle non-lethally he is forced to accept the offer of Lord Brisby. Brisby, who claims that he loves Lady Boyle, tells Corvo to knock out Lady Boyle and bring her to the cellar. Once Corvo has done so, he is awaited by Brisby who loads Lady Boyle onto his boat and disappears with her, claiming that she will learn to love him one day and has the rest of her life to do so. Brisby seems to have made good on his promise as after Lady Boyle has been eliminated non-lethally, the Outside states: "She supported a tyrant, the Lord Regent. And lived in opulence while the people of the city starve to death and live in fear of plague. Now she'll live out her days, month after month, year after year, far away, even as her fine clothes wear into tatters and her silken hair gets dull and gray. Plenty of time for reflection."
The developers later admitted via Twitter that they had been "wrongheaded" about the idea to give Lady Boyle to her stalker, and said that "she probably wrapped that pathetic adoring creep around her finger."
If one of the three sisters survives the party she later sends Corvo a letter of gratitude and a rune.
While infiltrating Dunwall Tower, Corvo can hear the Lord Regent lamenting Lady Boyles death/disappearance and regretting not having been present on the party to prevent it.