Complex sentence: However, the Silvan Elves would not place their army under the supreme command of Gil-galad and after the Battle of Dagorlad, Oropher led a reckless assault on Mordor in which he was slain.
Split into simpler sentences: However, the Silvan Elves would not place their army under the supreme command of Gil-galad. After the Battle of Dagorlad, Oropher led a reckless assault on Mordor in which he was slain.