Ztar

Ztars, also known as Shadow Stars in Mario Party 6, Dark Stars in Super Mario RPG: Legend of the Seven Stars and 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. They serve various functions from game to game, but usually take away one Star from the player's possession. Its name comes from a corruption of "Star," and is spelled with a 'z' instead of an 's.' 'Z' can be interpreted as a backwards 's,' referencing how Ztars are meant to be the opposite of Stars.

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 giant charcoal-colored stars with angry black eyes used in a powerful attack by bosses to crush one character.

Mario Party
Ztars reappeared in the first installment of the Mario Party series,Mario Party, where they are the bogus item for Mario's Rainbow Castle. Their appearance in this game was changed to a black star with white eyes. Bowser will sell the player a Ztar for 40 coins, and it does not have an effect on the player's star count.

Mario Party 2
In Mario Party 2, if the player lands on a Bowser space and gets the event called "Bowser's Multiplying Toads," Bowser will 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, will either make 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 appearance in Mario Party 3 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 lands on a Bowser Space and has at least one star, Bowser will occasionally give the player a Ztar, which will subtract one star from the player's current total. It is impossible to go below zero stars in Party Mode; however, in Card Party, players' star counts can go below zero if the Ztar Star card is revealed.

In Super Duel Mode, a Ztar can be seen as the power source for the Ztar Engine engine part.

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 given that the player has one or more Stars. They serve the same purpose as in Mario Party 5. 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 minor cameos in the comparison quiz section of Speak Up and in Tricky Tires.

Mario Party 7
In Mario Party 7, they were again renamed to Dark Stars (or Z-Stars in PAL versions). At one of the Green Spaces in Grand Canal, the player must choose a chest from a group after the chests have been 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.

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
Although Ztars don't appear in Mario Party 9, Mini Ztars do make an appearance, where they deduct the current amount of Mini Stars a character has.

Mario Party: Island Tour
As with Mario Party 9, Ztars do not appear in Mario Party: Island Tour. However, Mini Ztars return. They are found exclusively on the Star-Crossed Skyway board and have the same function as before, deducting Mini Stars when collected.

Trivia

 * There is an unused music clip from Mario Party 8 which is a variation on the "You Got a Star!" jingle, seeming to suggest that Ztars were originally going to appear in the game.
 * One of the main villains of Mario & Luigi: Bowser's Inside Story, the Dark Star, resembles a Ztar.