Complete the Gate Generator with Rudy rejoining the party and go to Saint Centaur and in the building under the Lone Paula Statue talk to the blind girl who will tell you about the Swordsman who has been visiting her the past few days. When you leave the city a message will tell you of a strange laughter and when you get the message 'Do you wish to continue?', choose 'Yes'. There will then be a short cutscene and then you'll be in bonus boss fights against 'Monster Zed'.