Dishonored: Death of the Outsider

You'll get this on Mission 4 (The Stolen Archive). Make your way to the top floor of the Royal Conservatory and the Curator's Office. I came through the elevator shaft and snuck along the top of the main atrium/lobby but there are probably other paths you can take. You'll come across and Oracular Sister telling a maskless Overseer (Brother Cardoza) who is dissecting a dead witch that Sister Rosewyn wants to see him. An objective marker will appear and give you Rosewyn's location. Use Semblance to take Cardoza's face and then go talk to Rosewyn. She will hand you a silvergraph plate during a brief cutscene. When the cutscene ends both this and Gnosis will unlock.

I just want to add an alternate strategy. When you come across the two Overseers talking in the courtyard kill the one that goes out toward the main entrance and then go Semblance the one that stops close to the basement window. Go in and immediately up the stairs and past a few more Overseers. At the top of the next staircase is an Overseer looking at a bookcase. Semblance him and then head across the second floor to the stairs. Carefully knock out the witch on the third floor and carry her out of patrol range of the other one. Alternatively, kill them both by using the first as bait. Carefully make your way down the hall and Semblance the witch by the bookshelf so that you can get into the next room and up the stairs. Cardoza is in the room by himself, giving you time to turn off Semblance and cast it on him after your energy recharges. Now just walk out to the waypoint, quickly get out of range of enemies after the cutscene so you're not overwhelmed.