Visions in the Fog

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

Quick walkthrough
Talk to Rictor. Go to spring and drink from it. Follow shadow figure. Kill ghouls at destination. Pick up note on table inside the shrine then read the side of a locker. Open door with password and grab the idol. Go back to Rictor

Detailed walkthrough
The Sole Survivor goes to speak with Brian 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.