Visions in the Fog

Visions in the Fog is a main quest in the Fallout 4 add-on Far Harbor.

Detailed walkthrough
The Sole Survivor goes to speak with Grand Zealot Richter outside of The Nucleus. He explains that to join the Children of Atom, the player must drink water from the sacred spring.

Journey to the spring and drink the water by interacting with it. The player will then enter a hallucinatory state, and a ghostly, dark figure (known as the Mother of the Fog) covered in black smoke will appear. Following the figure will eventually lead the player to a shrine, which is guarded by feral ghouls.

After eliminating the feral ghouls, enter the shrine. The marker will point to an icon located behind a maglocked security door that can only be opened with a password-protected terminal. To get the terminal's password, read the note on the shelf, which hints at the password's location on the side of a locker near a periodic table. Retrieve the icon and bring it back to Richter, who will allow you to join the Children of Atom and enter the Nucleus.

Behind the scenes
Every element in the MoThEr password for the shrine terminal (Molybdenum, Thallium and Erbium) has some connection with nuclear technology. Molybdenum is part of an alloy used to create missile parts, Thallium-201 is used in Nuclear cardiography, and Erbium is used as a control rod (item that absorbs neutrons) within nuclear reactors.
 * Molybdenum is listed as element number 99 in the shrine password, but it is actually element number 42 (element 99 is Einsteinium).
 * If Einsteinium is used instead, the word becomes "Esther" - the name belonging to a Fat Man from the Fallout: New Vegas add-on Gun Runners' Arsenal, continuing the nuclear theme.

Gallery
Туманные видения