The defeat of Sauron on Dagorlad was shattering but still he has survived and if he is not destroyed, all of Middle-earth will be ruined. Now Barad-dur the dark tower is besieged and you must break it and vanquish Sauron.
P.S:- If killing Sauron doesn't work just hit the tower with lots of rams and destroy the nearby empire, there is bug there! Watch out for the Lord of the Nazgul, he bites! Sauron is the idiotic Samurai warrior surrounded by rocks.