Grammatically, A has the phrase “That legendary place at the end of the map” as the subject of “reveals”, meaning that the legendary place itself reveals that “it’s only legendary until someone proves it real.”
B has “That legendary place” as the subject of the main clause, and adds “that the end of the map reveals” as a relative clause that provides more information about the legendary place. The end of the map reveals the legendary place, and the legendary place “is only legendary until someone proves it real.”