Zhaitan is an Elder Dragon capable of resurrecting the dead and corrupting them so they bend to his will. I don't want to give away too much of the story, but he is basically destroying the world, and everyone is counting on you (and your friends) to stop him and save everyone's asses. He is huge. Like, incomprehensibly huge. And you will get to fight him. Again, how could anyone not want to do this?