-( Story related, cannot be missed )-
The plague affecting Sei'an City, and the Emperor himself, is the spirit Blight that is possessing Goldnail. Journeying into the Emperor, you'll find this sword stuck on the platform and encounter the boss. When he starts shaking, use Veil of Mist to dispose of his ghostly aura and be able to attack him as he moves. When he summons a ring of swords, Power Slash the blue sword, and then attack Goldnail directly. Repeat these actions to defeat the evil spirit.