Complex sentence: The Valar, aware of the risk Melkor posed to the Elves, attacked him in Utumno, and after a long siege, Melkor and his forces were defeated, and Melkor was taken prisoner to Valinor to serve "three ages" in the Halls of Mandos.
Split into simpler sentences: The Valar, aware of the risk Melkor posed to the Elves, beleaguered him in Utumno. After a long siege, Melkor and his forces were defeated, and Melkor was taken prisoner to Valinor to serve "three ages" in the Halls of Mandos.