Ultra Sword is an upgraded version of Sword. Unlike other Super Abilities, the Ultra Sword is also used to finish off two bosses - Grand Doomer and Magolor's first phase (as well as their EX versions). It also is used in the penultimate level of the game to defeat every mid-boss (except Super Bonkers), obliterating each in just one chop.
In Kirby Star Allies, Magolor can utilize an Ultra Sword in his Dream Friend moveset. His has a purple handle as opposed to Kirby's light blue one, and the central symbol is an Energy Sphere instead of a star.
In the Super Smash Bros. series
Ultra Sword appears as Kirby's Final Smash in Super Smash Bros. for Nintendo 3DS and Wii U and Super Smash Bros. Ultimate. It replaces Cook, Kirby's Final Smash from Super Smash Bros. Brawl. When used, Kirby will change his appearance to that of the Ultra Sword Super Ability. He will slash at his opponent several times in quick succession as they are unable to move, before performing a finishing blow and sending them flying.