To start the quest talk to the shogun and ask him about any quest to do for him, he will say you that he needs to send a message about the war with Oshurakan to the emperor which resides in the city of Kitaroyama, the capital of the kingdom of Chinyang, say him that you can deliver the message and he will give you the message, then you must go the dock and and find a person with a samurai armour, talk him and he will ask you that if you want to go to Koroshima for a price of 100 coins give him the money and you will sail to the town of Koroshima.