Story-related and unmissable. The One Reborn is the final boss of Yahar'gul, Unseen Village. He is one of the easiest bosses in the game, since he is rather slow and just does not deal a lot of damage. There will be some mages up in the nearby towers casting fireballs down at you, so you can optionally go up the towers and kill them if you want (though this is not necessary).
As for The One Reborn itself, just stay close to it and underneath its rear end. It will barely be able to hit you when you're back there, but even when it does, it won't do much damage. See the following video guide: