From the Super Mario Wiki
The Vacuum Helmet is a device used by Fawful in Mario & Luigi: Superstar Saga and Mario & Luigi: Bowser's Inside Story. It is used to get Fawful around and attack the Mario bros. It is also noticed to have many functions and is Fawful's most used invention.
Mario & Luigi: Superstar Saga
In Mario & Luigi: Superstar Saga, Fawful uses the Vacuum Helmet to suck up Princess Peach's voice before the onset of the game. He uses the Vacuum Helmet's fireball ability to battle Mario and Luigi when they come to reclaim Princess Peach's Voice; he then uses the attack to destroy Bowser's Koopa Cruiser. In battle it has 18 HP.
Later, when Cackletta is defeated by the Mario Bros., Fawful sucks her soul up with the Vacuum Helmet and deposits it inside Bowser's unconscious body, thus saving her from death. During the final battle of Superstar Saga, Fawful and his Vacuum Helmet are seen in one of Cackletta's attacks, but in ghost form. In the attack, the Vacuum Helmet will shoot fire balls at both bros. If Fawful is in front of Mario, the Vacuum Helmet will first shoot at Luigi first, then Mario. On the other hand, if Fawful is in front of Luigi, the Vacuum Helmet will shoot at Mario first.
Mario & Luigi: Bowser's Inside Story
In Mario & Luigi: Bowser's Inside Story, the Vacuum Hemet is used by Fawful to suck the power out of the Dark Star and become Dark Fawful. Afterwards the helmet is fought in the battle against Dark Fawful where Bowser can inhale it and the Mario Bros. will fight it from inside him. In this game, the helmet is less important, although it is much more life-like than before. So far, the helmet's last appearance was in Bowser's Inside Story.
The Vacuum Helmet has many functions, and is primarily used by Fawful to fly around or hover. The Vacuum Helmet's nozzle can both shoot fiery balls of energy at Fawful's opponents and vacuum up other things, which are then stored within the Vacuum Helmet.
Mario & Luigi: Bowser's Inside Story introduces more functions, including the Vacuum Helmet's ability to function independently from Fawful. It is also revealed that the Vacuum Helmet's nozzle can morph into a kind of basket, inside of which Mario or Luigi can be trapped and drained of their HP. Fawful can also fly using the helmet. Another attack is charging up energy balls up the tube. The player will have to look at the color of the helmet. Red is Mario while Green is Luigi and the player counters with hammers. While Fawful is fighting Bowser, he may use his Vacuum Helmet to avoid some of Bowser's attacks or he can revive some of his own HP. The Vacuum Helmet also morphs its nozzle into what looks like teeth, allowing Fawful to grab onto objects and swing from them. At some point, the Vacuum Helmet will fly above Fawful's head on the top screen, where Bowser can use his vacuum ability to suck it up into his body, where the Bros. will fight it. After taking a certain amount of damage, the Vacuum Helmet will explode and cease to exist, making its last appearance. However, if Bowser's power is high enough, Dark Fawful can be defeated without his headgear being inhaled. After he is defeated, the Vacuum Helmet will run away and never appear again.
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