Ztar is an evil star imbued with dark power created by SMG3 and Bowser. It is the main antagonist of Bad Stars and Bad Star's Back! 300k special. It is one of the foremost antagonists of the series, due to it's sheer destructive malice and immense dark power.
In "Super Mario 64 Bloopers: Bad Stars" SMG3 was taking a nice evil walk, when SMG4 chucked a book out the window and hit him. The book told a story about an evil star, giving him an idea. So he went to Bowser to create it... but a Toad appears out of nowhere and runs into the Star while it is being created and fuses with it. When SMG4 sees the Ztar, he demands the two idiots fix the problem, so they make a good star to defeat it. The Good Star and the bad star duel, but Ztar runs for it. So SMG4 and Mario shoot Toad at it, making it explode. The Ztar exclaims they can't kill him because he is immortal, so the blooper ends with the Ztar tied up on a boat with the good star, floating away.
It reappears in "Sonic the Derphog: The Evilness of Eggman (?)" in which he wears a bikini and a ribbon (showing he's gay) in which he tries to be sexy to SMG3, causing SMG3 to be in shock and burn his eyes.
It also makes a cameo appearance in "SM64: The Retardness of 2014".
It plays a major role in "SM64: Bad Star's Back" where it frees itself from its original boat trip 10 months after he was sent on it. He proceeds to cause mayhem, causing a secret service to kidnap Mario and SMG4 to save the world. The two confront the bad star several times before it is defeated by the combined efforts of Toad, Mario and the Good Star. He was last seen going into a portal created by the Good Star where they both disappeared along with a plate of spaghetti.
It makes a cameo appearance in "Sonic the Derphog: The Item Brick" as an item given by an Item Block from (from the Mario universe) that is the major object of the plot.
It also makes another cameo in "The Wacky Wario Bros.: Golfing for Gold" with Toad when the golf ball launched by the Wario Bros. flies in space.
It also makes a cameo in Ssenmodnar 9 (Time travel edition) when Starman3 is sparkling before loading the pingas launcher Ztar can be seen floating over his head for a couple of seconds