The exclusive phenomenon in the Paldea region of Pokemon Scarlet and Violet is known as Terastalizing. This is the Generation IX battle gimmick, such as how previous entries employed Dyanamax, Z-Moves, and Mega Evolutions.
Terastalizing does a bit of everything the prior ones did by adding a new move, changing the appearance, and altering the Pokemon's type.
Trainers can choose to Terastalize their partner in a Pokemon Scarlet and Violet battle. They will crystallize, put on a fancy hat, and have a new type that can easily turn the tide of battle.
What is Terastalizing in Pokemon Scarlet and Violet?
The phenomenon known as Terastal is conducted by Terastalizing a Pokemon. It only occurs in the Paldea region. Pokemon that Terastalize gain a Tera Jewel atop their head and begin to shine and glisten like a gem.
Terastal energy is said to seep from the ground of Paldea and is what causes the phenomenon. Professor Sada and Professor Turo, from Pokemon Scarlet and Violet, respectively, are focused on researching Terastal.
How to Terastalize in Pokemon Scarlet and Violet?
The exact mechanics won't be known until the games release, but Game Freak has confirmed that a new item will be needed to reach the Tera form. That item is known as the Tera Orb.
Only select trainers are given a Tera Orb, which will certainly mean the main character that players control. The Tera Orb can be used to Terastalize a Pokemon, but then it needs to be charged before reuse.
Trainers will be able to charge their Tera Orb by touching crystals that emit Terastal energy or by visiting a Pokemon Center. The option to use it in battle will appear if it is fully charged.
What does Terastalizing do?
Aside from simply giving the Pokemon a diamond-like appearance, Terastalizing provides some powerful bonuses. All Pokemon can Terastalize and have any of the 18 types as their Tera Type.
The Tera Type is inactive until the Terastal phenomenon is enacted. An example of this was shown in a recent trailer.
Chansey is typically a Normal-type Pokemon. In the trailer, trainers battled a Terastalized version in a Tera Raid. Instead of being a Normal-type, this Chansey's Tera form was a Ghost-type.
This change in type can be strategically used to give a Same Type Attack Bonus to a move that usually wouldn't have one. Coupled with that, it can also change a Pokemon's weaknesses to withstand hits that might have been supereffective without the Tera Type.
Pokemon that use a move that matches their Tera Type and one of their original types will receive an incredible boost. This will make it just as valuable to have a Tera Type that doesn't shift from one of the Pokemon's actual types.