Complex sentence: The Siege of Barad-dûr lasted for 7 years, during which Anarion was killed, Sauron emerged from his tower and engaged the Alliance forces personnally.
Split into simpler sentences: The Siege of Barad-dûr lasted for 7 years, during which Anarion was killed. At the end Sauron emerged from his tower and engaged the Alliance forces personnally.