From the Super Mario Wiki
Mini Ztars are small, dark purple miniature Ztars, introduced in Mario Party 9. Mini Ztars serve as the opposing counterpart to Mini Stars as they will causes the players to lose a certain amount of Mini Stars (in other words, gain Mini Ztars) after coming into contact with them.
Mario Party 9
As Mini Ztars are found in groups around the boards around certain spaces, they immediately display their effects if the player runs into them while presuming the role as the captain. Mini Ztars are rarely found alone, and are often found in groups of either three, five, or ten.
Mini Ztars can also be obtained in the "Unlucky" pipes as these miniature stars will be given out in a number depending on the player's dice roll. Unlucky routes may also lead to the player collecting Mini Ztars.
Mini Ztars are replaced by dark, purple bananas called Z-Bananas in DK's Jungle Ruins, though they have the same effect; they cause the player to lose Bananas when coming into contact with them. They can be found as singular bananas or in bunches.
Mario Party Island Tour
Mario Party 10
Mini Ztars reappear in Mario Party 10 the direct sequel to Mario Party 9, with similar gameplay. They are considerably rarer.
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