Ztar

Ztars (also known as Shadow Stars in Mario Party 6, Dark Stars in the North American version of Mario Party 7, and Z-Stars in the PAL versions of Mario Party 7) are black counterparts to Stars found mostly within the Mario Party series. Instead of granting the player a Star, it takes away one in the player's possession. Its name is a corruption of "Star" (spelled with a 'z', which can be interpreted as a backwards 's'. 'Z' being a "backwards 's'" reference the Ztar being the opposite of a regular Star).

Super Mario RPG: Legend of the Seven Stars
The first appearance of Ztars was in Super Mario RPG: Legend of the Seven Stars where they are used as a powerful attack by bosses to crush one character with a giant black-colored star.

Mario Party
Ztars reappeared in Mario Party, where they the Bowser Bogus item for Mario's Rainbow Castle. Bowser will sell the player a Ztar for 40 coins. In this game, it does not count as a negative star.

Mario Party 2
In Mario Party 2, if the player landed on a Bowser space and got the event called "Bowser's Multiplying Toads", Bowser would create two Toads. One of them is real and one of them is Koopa Kid in disguise. If the player runs into one it will offer a "Star" for 20 coins and, once paid, either a Star or Ztar will appear. If a Ztar appears, it won't count to the amount of Stars the player has so twenty coins are wasted. However the other Toad will still offer a real Star.

Mario Party 3
Ztars make a cameo appeareance in the minigame Cosmic Coaster. The star on the goal looks like a Ztar until a team reaches it. If a team was close to the goal, but lost the minigame, the Ztar makes a falling animation.

Mario Party 4
Mario Party 4 features a Ztar hosting the volleyball game in Thwomp's Backroom Ball.

Mario Party 5
In Mario Party 5, if a player landed on a Bowser Space and had at least one star, Bowser would occasionally give the player a Ztar, which would subtract one star from the player's current total. It is normally impossible to go below zero stars; however, in Card Party, players can go below zero if the Ztar Star card is revealed. Plus, in the Super Battle mode, the Ztar is an optional engine type where it is the power source.

Mario Party 6
Ztars were renamed to Shadow Stars during the events of Mario Party 6. In Clockwork Castle, if Bowser encounters a player, he will give the player a Shadow Star if the player has one or more Stars. This will cause the player to lose one of his or her Stars. If the player has no Stars, Bowser will give them a Special Shadow Star, which takes away Coins instead. Bowser will also give the player a Shadow Star if they land on Bowser's Battle Yacht in Castaway Bay. Shadow Stars also make a minor cameo in the comparison quiz section of Speak Up.

Mario Party 7
In Mario Party 7, they were again renamed to Dark Stars. In one of the Green Spaces in Grand Canal, the player must choose a chest after the chests are randomized by a Blooper. One of the chests holds a Dark Star. If the player chooses this chest, they must take it. In Neon Heights, at Bowser Time, Bowser sometimes puts a Dark Star in one of the chests. If the player pays Koopa Kid for this chest, they will be forced to take the Dark Star inside.

Players who get the Dark Star will lose one Star, which can heavily turn the tables in certain Solo Cruise events.

Mario Party DS
In Mario Party DS, they were renamed Ztars again, and occasionally appear in Hidden Blocks. If any players are unfortunate enough to get one, they will lose one Star. Also, a figure called Ztar appears at the same time the Star figure appears.

Mario Party 9
In Mario Party 9, Mini Ztars make an appearance, where they deduct the current amount of Mini Stars a party member has.

Trivia

 * One of the main villains of Mario & Luigi: Bowser's Inside Story, the Dark Star, looks like a Ztar.