In Rick and Morty S04E05 - "Rattlestar Ricklactica", Rick and Morty had this conversation:
Morty: I'm dying, Rick. [Groans] I can see the black mountain.
Rick: People who are really dying don't keep bringing it up.
Morty: Is that true?
Rick: I don't know. I'm just usually around people that die faster.
I haven't heard of "black mountain" being used as an expression or idiom for dying. Is "black mountain" a reference to something?