The title story of Harlan Ellison's 1975 collection Deathbird Stories, and arguably his most powerful and significant work. The story first appeared in the March 1973 issue of The Magazine of Fantasy and Science Fiction and later won the 1974 Hugo for Best Novelette. If you ever thought the serpent in Genesis got a bad rap, you should read this story.
The premise of the story is this: thousands of years ago the Earth was created by a beautiful and intelligent race of creatures who were driven out by the coming of a cruel and insane being. One of them, Dira, was left behind to give the inhabitants of Earth wisdom, so that they might someday be able to defeat "the mad one." As the story begins, Dira, the Snake, is guiding Nathan Stack, the last reincarnation of Adam, across a ruined, post-apocalyptic Earth toward the lonely mountain where he will have his final showdown with God.
The story is told in an unusual, almost postmodern way: 26 sections that include the main narrative, excerpts from the Book of Genesis and Nietzsche's "Thus Spake Zarathustra," a personal essay Harlan wrote about his dog Ahbhu, and academic-style lists of questions. The patchwork of styles works: what could easily be didactic and obvious instead becomes profound. You are left to decide the story's ultimate meaning and relevance for yourself.