"In honor of our fathers" - "One Khan, One Clan" [Campaign 3, Mission 2]

You need to go around the last Castle and summon the ship from there using a spell (from the amulet). If you want more details, check pages 1-2 of the "Tribes of the East" add-on walkthrough topic. There are even screenshots posted there (message #33). Everything is explained very clearly and in detail!