The final battle at the end of the end times in Arda as prophesied by Mandos, to be followed by Eru's second greater music.

We are expecting Ar Pharazon and the Númenórean Fleet (possibly Spell Jammers or Ghost Ships), the return of Sauron (possibly through the Cult of Sauron obtaining the Crucible and Avatar of Sauron and using the Crucible to put Sauron into the Avatar), the return of Morgoth, the return of Ungoliant / Lolth, and a sprinkling of significant evils that followed us here from other spheres.