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.
Magic Box Artifact's
Magic Box Artifact's are personal magic boxes that belong to the Great Powers. The Boxers copied the idea to lock away more malign powers. According to Dunster Happ, because not much can harm them, the Powers use these boxes as a way to die or commit suicide. The boxes are hidden away and can only be accessed by their owners. It is theorized that they pass on to an afterlife of sorts. It is shown that a Great Power can be killed by another's box artifact when Mister Dark was pulled into The North Wind's artifact.