The side quest The Mistress of Mystery starts right after the Mystery Pioneer side quest. Find out how to complete this quest on the Guide Fallout 76.
Return to the Riverside Manor (picture1), go down to the basement and use the Cryptos terminal (picture2).
Log in to the account of Mistress Olivia Rivers (picture3), choose “Administrator actions”, “Allow access”, again “Allow access”, then choose your username (pictures4and5).
Now enter the office of the Grand Mistress by interacting with the palm scanner (picture6). Use the terminal (picture7), and read the different entries (picture8).
Go back to the mansion and use Olivia’s terminal (picture9). Read the personal journal dated 30/09/77 (pictures10and11).
Now head to the meeting place (picture12), and pick up the items on the corpse of Shannon Rivers (picture13).
Go back to the basement of the Riverside Manor and use the Crypto Terminal (picture14). Log in to the account of Grand Mistress Shannon Rivers (picture15), select “Administrator Actions”, “Authorize Promotion”, “Mistress of Mystery”, and finally select your username.
Last step, log in to the Cryptos terminal using your credentials to become the new Mistress of Mystery (pictures16and17).