Complex sentence: It was named after Manwë, the fictional king of the Valar in J. R. R. Tolkien's Middle-earth legendarium, Manwë is foremost among the deities who rule the world.
Split into simpler sentences: It was named after Manwë, the king of the Valar. In J. ~ R. ~ R. Tolkien's mythology, Manwë is foremost among the deities who rule the world.