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This article contains plot spoilers. If you do not yet know of this information, it is recommended that you read at your own risk or not at all.
In the room adjacent to the black market, there is a brush, a palette, and a severed left ear. This could be a reference to Vincent Van Gogh, who is said to cut off his own left ear. [Credit: u/adamsmith9312]
In the basement of the Royal Conservatory, you can find a note, detailing the possessions of a witch. Among them was a waxwork doll, fashioned to look like Kirin Jindosh, and it had been struck with several hairpins in the neck, head and groin. These are the same body parts that were attacked when the protagonist assassinated Jindosh. [Credit: Капитан Майк Фоули]
Markings similar the plague marks in Dishonoed, are found throughout the Conservatory.
Since the witches lost their powers, this one had to use pistol to defend herself. And the bottles suggest that she was drinking heavily while awaiting her inevitable death.
Oracular Sisters use heavy attacks that deal heavy damage and would stagger even if you managed to block, similar to the protagonists' charged attack.
Oracular Sisters can expertly deflect projectiles with their maces. [Credit: Annibale Porfiry]
In the room with the Oraculum, you can knock down the censers and the dead Oracular sisters would start reciting a number of prophecies. The most notables are:
"Fine bones and fine threads of hair. She keeps both close. It is dead, yes, long dead. It is repulsive heresy." This could refer to the Rat Talisman that Billie carries.
"Tell them to cull the rats. Place poisons in the streets. These vermin carry secrets and disease." Since this came from dead Oracular Sisters, it could be an old prophecy that predicted the Rat Plague in Dunwall. Or this could mean the sisters knew about Billie's ability to listen to rats and attempted to hinder her. [Credit: Pauolo]
In addition, one of Lena Rosewyn's prophecies says "Five red strings pulled taut over some unknown instrument. My sisters could not make sense of it. Perhaps it will reveal itself again." This could either refers to:
The Oraculum due to the 5 red strings stretched out over its plan. [Credit: Pauolo]
Or Billie's Black Shard Arm because if you look closely, there are bright red strings inside the five fingers.
There are many details that somehow connect the Abbey to the Cult of the Outsider:
Oracular Sisters wield maces which, in Harvey Smith's words, ended with "a single open eye". This is possibly a depiction of the Eye of the Dead God.
If you manage to initiate a conversation with Lena Rosewyn without alerting any of the nearby Oracular Sisters, she will say that she foresees Billie killing the Outsider, and that she cannot let her "destroy what gives [the Abbey its] purpose". This implies she knows that destroying him is possible, a knowledge that is closely guarded by the Cult of the Outsider.
On the same note, during battle Rosewyn would say "We will keep that knife away from prying eyes when you are dead" and "You will never reach the Void". This clearly hints that she knows of the Knife and that the silvergraph points to the entrance of the Void - yet more closely guarded secrets of the Cult of the Outsider.
The white-red-black uniform of the Oracular Sisters strangely resembles the Cultists', and their transformation into the Envisioned. The gold flakes over black fabric on their arms and torsos look similar to parts of the Envisioned. The blindfolds symbolize the sisters' blindness - as if they are "eyeless" or no longer receive blessing from the Eye of the Dead God - as opposed to the Cult who sometimes call themselves "We of the Eye". A conversation between two cultists reveals that they use the term "eyeless" to refer to those who are not considered "enlightened", sometimes even themselves (and that's probably how the name "Eyeless gang" came to be).
The color of the Sisters' blindfolds are also similar to the Cultists' ties and scarves.
The gold flakes resemble the gold lights/veins on the Envisioned's spike arms and chests.
In addition, there are evidences of the Abbey using magic, that is, powers coming from the Void:
The Oracular Sisters can predict the future.
During the conversation between Billie and Rosewyn, Rosewyn would say that "Every hour, our sight grows short. Only I have witnessed the method of our blindness. Only I have seen that knife in a dead boy's heart." This suggests ending the Outsider would end their foresight. [Credit: u/Kaseven7, u/xbully1994x]
Another prophecy coming from the Oraculum seems to substantiate the above point: "What is this darkness? What is this end to all sight?"
They also can clearly see even though blindfolded. Not only that, they can easily deflect projectiles as mentioned above. This implies they have some kind of powers similar to Dark Vision and Reflexes.
Breanna Ashworth claimed that the Oracular Sisters minds are linked together, even through death.
Overseer music boxes employ what they call the "ancient music", which is supernatural in nature. In the Edge of the World mission, an Overseer in the Karnaca enclave said that "The techniques [of making the music boxes], if applied by anyone but the most faithful, resolute Overseers, would border on heresy."
When Mesmerized by Emily's Void spirit, Liam Byrne says that "I saw something like this once, during the Trials of Aptitude." If this is true, the Overseers used a similar power as part of the trial.
The book Metaphysika Mysterium outright states that at least one member of the Abbey harnessed the powers of the Outsider by "using the flesh of others". We now know that Paolo possesses a supernatural ability thanks to Granny Rag's marked hand, and the book Dishonored Roleplaying Game also confirms that a severed Marked hand grants a fraction of the powers of the original Mark bearer. [Credit: u/Kaseven7]
There are at least six different ways you can acquire the information from the archive and each has a different outcome:
You can assume Cardoza's identity, talk to Rosewyn, and she would willingly give you the silvergraph.
If you let Cardoza take the silvergraph, you can follow him to a projector and listen to his monologue. This way you can learn the information without needing to take the silvergraph.
After the two above outcomes took place, if you go and talk to Rosewyn, the conversation is slightly different in which she says "Brother Cardoza is taking care of that silvergraph".
If you managed to steal the silvergraph from Rosewyn without alerting anyone, the conversation between Cardoza and Rosewyn would still take place but they'd quickly realize the item's gone. Cardoza would run to down stair and alert the overseers and oracular sisters via the loudspeaker system.
Similarly, you can steal it from Cardoza after he took it from Rosewyn. He will realize that when he reaches the silvergraph projector, and also runs to a nearby loudspeaker mic to alert his allies.
If you eliminated Rosewyn and hid her body without alerting anyone (a hook mine and Quiet Hook bonecharm would do), Cardoza would come, find no one, and go back to his office.
You can of course kill Rosewyn to get the silvergraph.
If you stand close to Rosewyn without alerting anyone, she will say a few amusing lines:
"I hear her footsteps in my mind."
"Am I stalked by my own vision?"
"Her hands are trembling. We know what she will do."
"Yes, I feel the fetid whispers of the Void along my neck."
"It comes closer." After this, she stands up becomes alerted.
