Baron's Bugle is a unique item in Dragon Quest IV that allows the full party to participate in the final battle. The holder merely has to blow the bugle to summon the rest of the wagon party which would normally be absent after Necrosaro's Castle. It has to be used outside the molten cave of Nadiria.
The bugle is found in a shrine to the east of Azimuth. The party need to lower the elevator to the lowest floor, climb back up to the top floor and then fall down to the top of the elevator to reach the chest.