Boimler looking at the black mountain
The black mountain was, according to Shaxs, a "spiritual battleground" where the soul went after death. There they must fight three faceless apparitions of their father, after which the surviving father makes them eat their heart.
In 2381, a resurrected Lieutenant Shaxs described his experience at the black mountain to Ensign Sam Rutherford. (LD: "We'll Always Have Tom Paris")
Later that year, when Lieutenant Commander Steve Stevens was brain dead for ten minutes after being turned to stone by a psychic mine on Jengus IV, he said that he saw "a koala sitting on a black mount[ain]". (LD: "Mining The Mind's Mines")
Later still, when Lieutenant jg Brad Boimler briefly died after being caught in an explosion from a power relay on Corazonia, he found himself in a mysterious room where the black mountain was visible from the window. (LD: "In the Cradle of Vexilon")
When Badgey ascended, he suggested that he would check out the black mountain. (LD: "A Few Badgeys More")
The black mountain was also referenced as a place after death in Rick and Morty, on which Mike McMahan works as producer and writer.
It was also referenced in kind in "Solar Opposites" where Mike McMahan works as the executive producer and head writer / showrunner.