Bluebeard loaned the use of his Swan Boat to the refugees trying to escape through the portal behind the Keep at World's End.
The portal was found after traveling down a rapid river, and just beyond the clouds following a waterfall. The boat comes equipped with sprouting wings, making it the only vehicle on hand that could reach the portal.
It's current status is unknown, likely hidden away somewhere in Bluebeard's vast apartment.
Links and References