Persistant medieval belief that just out there beyond the furthest extent of known geography was a vast Christian kingdom ruled by the priest-king Prester John. This was initially assumed to be in India, then China, later in Africa, getting shuffled by cartographers to whatever part of the world hadn't been explored yet. In 1221 Crusaders found themselves aided in their fights against the Moslems by a mysterious army who they assumed to be from the kingdom of Prester John. But it was only Ghengis Khan.