The Stellar type was introduced in Pokémon Scarlet and Violet The Indigo Disk to go along with Gen IX’s Terastallization mechanic. One fan theory, however, suggests the unique type was already being hinted at back in Legends: Arceus.
Stellar is arguably the weirdest and most complex of the 19 types in the Pokémon franchise, being locked behind Terastallization and having only two moves—Tera Blast and Terapagos’ signature attack, Tera Starstorm. While the Stellar Tera Type does nothing for the user from a defensive standpoint, it makes Tera Blast and Tera Starstorm super-effective against any Terastallized target. Any Pokémon can use the Stellar Tera Type, but it’s mostly marketed toward the Legendary mascot of The Indigo Disk, Terapagos.