What phenomenon debuted in Pokémon Scarlet and Violet?

Terastallization debuted in Pokémon Scarlet and Violet.

Terastallization is a battle phenomenon associated with the Paldea region. When activated, a Pokémon crystallizes and displays a Tera Jewel above its head. Its Tera Type determines the type-based advantages and changes available during the battle.

Every Pokémon has a Tera Type, although it may differ from the Pokémon’s usual type or types. This creates new tactical possibilities: a Pokémon can gain resistance to attacks, strengthen moves of a particular type, or surprise an opponent with an unexpected defensive matchup. The effect generally lasts until the battle ends or the Pokémon is unable to continue.

The games were released for Nintendo Switch in 2022. Terastallization followed earlier generation-specific mechanics such as Mega Evolution, Z-Moves, and Dynamax. Those systems are not interchangeable, even though each temporarily changes how selected Pokémon perform in battle.