From the Super Mario Wiki
The Bob-omb Mafia is a large group of Bob-ombs which Big Bob-omb leads. They make their only appearance in Mario Party Advance. While traveling through Shroom City, the player encounters several people who mention that the mysterious leader of the Bob-omb Mafia is located in a secret room somewhere in Shroom City. Going to a hidden floor in the Mushroom Condo, the player finds Big Bob-omb and challenges him to a game of chance. Earning the respect of Big Bob-omb, the player gains a minigame and Gaddget.