Ztar

A Shadow Star (aka Ztar) is a black star found in the Mario Party series.

In Mario Party, the Shadow Star, back then known as Ztar, is the Bowser Bogus item for Mario's Rainbow Castle, sold for 40 coins. In Mario Party 4, it hosts the volleyball game. In Mario Party 5, getting one counts as a negative star. In Mario Party 6 the Ztar is renamed Shadow Star. Bowser gives you a normal Shadow Star when you meet him with one or more stars in possession. The normal Shadow Star is in Mario Party 5. If you don't have any stars when meeting Bowser, he will give you, as he refers to it, his special Shadow Star, which steals 20 coins. During Mario Party 6, in Clockwork Castle if Bowser goes on the space you're on, he gives you a Ztar and takes all your Stars away. If you don't have any Stars he takes away all your Coins.

Trivia

 * A black star also appears in Super Mario RPG: Legend of the Seven Stars when the attack Dark Star is used.