Complex sentence: King used the "relationship" between him and Bachman as a concept in his 1989 book "The Dark Half", a story in which a writer's darker pseudonym takes on a life of its own.
Split into simpler sentences: King used the "relationship" between him and Bachman as a concept in his 1989 book "The Dark Half". In the novel a writer's darker pseudonym takes on a life of its own.