The Eternal Champion (Eternal Champion Series #1)

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The Eternal Champion is a fantasy novel by Michael Moorcock. First published in 1970, it is based on stories Moorcock published in Avillion and Science Fantasy. It is the first in a trilogy of books about the Eternal Champion in his incarnation as Erekosë. The sequels are Phoenix in Obsidian, also published as The Silver Warriors, and The Dragon in the Sword.
In the book, John Daker is taken from his ordinary life in the modern world and incarnated in the form of Erekosë, a long-dead hero. He learns that he has been summoned to lead the human race in a fight against the alien Eldren, as he once did in the legendary past. He must balance his 20th-century consciousness with his patchy memories of a past life as a sword and sorcery-type warrior. He also must learn to distinguish the truth, since the rules of his new world are not necessarily the rules of Earth, and he cannot decide whether reports of magical events are real.
Although it lacks the popularity of other books by Moorcock, especially the Elric series, The Eternal Champion is a key text in the mythology of The Multiverse.