‘Acid Rain’ is the 10th and the last track on ‘Hail to the King’ the sixth album by Avenged Sevenfold.

Vocalist M. Shadows breaks it down in this article

There’s another song called “Acid Rain,” which follows on from Planets. It’s the love story of what happens after the planets collide and you get cast into space with your loved one to die. So there are a bunch of dark, crazy stories that are really all over the place, but which are definitely metal.