The Masayuki map is a specific treasure map in Dragon Quest IX. It is named for the Japanese player who randomly received it in Dragon Quest IX. The map became a national sensation in Japan, being traded thousands of times all over the country via Tag Mode. The name of the map is Ruby Path of Doom Lv. 87. The boss is Tyranosaurus Wrecks.
The map is famous for its high number of metal enemies on the final floor of its grotto, yielding opportunities to earn massive amounts of experience very easily. While every enemy that appears on the fifteenth floor is a metal king slime, other monsters may appear on the start of the battle. View details of the map here. (Site is Japanese!)