Ztar is a black star owned by Bowser, and various other characters.
Appearances
Mario Party
Bowser owns it in Mario's Rainbow Castle as one of his Bogus Items. He may sell it to the player for 40 coins. It is relatively useless in this appearance.
Mario Party 2
If the wheel on Bowsers space hits an event called "Bowser's Multiplying Toads", a Toad clone would appear, if you land on the fake Toad, it would reveal that he is actually Bowser Jr. as he gives you a Ztar, but it doesn't take away your star, but has no effect on the Star count.
Mario Party 4
Ztar appears in the background on the boards Thwomp's Mega Board Mayhem and Thwomp's Mini Board Madness as extras. It flies in a circle and is not important. Later in the game, Ztar challenges the player to a game of beach volleyball.
Mario Party 5
However, the black star's effect was first introduced until in this appearance, where it mainly appears in the Card Party. Sometimes when a Star Card is reached this star will appear, referred to as a Ztar Star. It makes the player lose a star, If the Ztar stays for a while in the card, It may get brighter, and suddenly it reveals a Star. Or sometimes it shows a small "Yellow Glow" but it turns black again. Ztar may also appear from a Star, the star when it stays in the card, it starts growing bigger, if it grows too much, it starts shaking, and it explodes, Revealing a Ztar Star. Bowser may also award it to a player as a "Star Present", It first resembles a glowing, Yellow Star, But it starts shaking, and it explodes, Revealing a Ztar (As said before, this may also happen with normal Star cards). He can also give it away on a Bowser Space.
Mario Party 6
Ztar appears as the Shadow Star. Here, it once again counts as a negative star. It is given to the player by Bowser on Bowser's Battle Yacht in Castaway Bay. He also gives it to the player on Clockwork Castle where it even has a space known as the Shadow Star Space. Here, the 20-coin Shadow Star is also introduced, given to the player by Bowser if they have no Stars. It makes the player lose 20 coins.
Mario Party 7
A Blooper in Grand Canal may also give the player a Dark Star, as it is called in this appearance. At a ? Space at the top of the board, it is put in a chest and recycled with other chests with various prizes. If the chest with the Dark Star is chosen, the player will lose one star.
Mario Party DS
Ztar makes a small appearance in Mario Party DS. In the game, a Hidden Block may summon one of these, causing the player to lose a star. It also appears as the penultimate figure.
Cameo Appearances
Mario Party 3
In the mini-game Cosmic Coaster, if one of the teams get the Star in the goal first and the other team doesn't get it but they get close to it, the losing team can see the Star in the goal as a Ztar falling to the ground. Also, before reaching the goal, the star looks like a Ztar.
Super Mario RPG: Legend of the Seven Stars
This game marks the debut of Ztars. The Ztar only appears in the attack Dark Star which is used by certain bosses (most notably Culex). The Ztar bounces on the screen and damages the target.