Pokemon Scarlet and Violet's third trailer came with an in-depth commentary about the Paldea region, which shed some light on several mysteries surrounding Gen 9 until recently. Three new Pokemon were revealed in Paldean Wooper, Fidough, and Cetitan, and The Pokemon Company also showcased the new battle gimmick for Pokemon Scarlet and Violet. This mechanic is called Terastallization, which allows trainers to use a special item to change their Pokemon's type to a corresponding Tera type.
The latent Tera type can be any of the standing 18 Pokemon types in the series. Pokemon Scarlet and Violet's emphasis on Terastallization is great for team variety and versatility while battling, as trainers can Terastallize their Pokemon for offensive or defensive purposes, strategically changing their type on the spot. However, there are some instances where the Terastal phenomenon can cause problems due to any Pokemon being allowed to have any type. The main culprits are Abilities or moves that have to do with Pokemon types to any degree.
Shedinja Could Become Immune to Damage When Terastallized
One of the most interesting cases for the Terastal phenomenon is probably Shedinja, the only Pokemon to have an Ability called Wonder Guard that makes it immune to non-supereffective damage. Shedinja's gimmick is being a Ghost/Bug Pokemon with only 1 HP, meaning it is immune to 13 types out of 18, but the remaining types will one-shot it if they hit. Via Terastallization, a Shedinja can change its type to make it impossible for opponents to damage it with certain moves.
An item called Air Balloon makes its user immune to Ground-type moves at the cost of being removed when the Pokemon is hit by any other move. If Shedinja and Air Balloon are both in Pokemon Scarlet and Violet, the unique Pokemon could Terastallizing into an Electric-type with Air Balloon, effectively becoming immune to all 18 types. In that case, the only way to defeat Shedinja would be via Sandstorm, Hail, moves that deal no damage but apply either Poison or Burn, and field moves that damage Pokemon upon entering the battlefield.
How Type-Changing Moves and Abilities Could Work With Terastallization
Type-changing moves and Abilities can be tough to work with when trying to Terastallize a Pokemon, or while fighting a Terastallized creature, because of the stacking interactions that can occur. For example, a Fire/Flying Pokemon that uses Burn Up and Roost will loose its types completely, becoming typeless until ts next turn when Roost's effect ends. A weird case would occur if Pokemon Scarlet and Violet feature Pokemon with powerful Hidden Abilities such as Protean and Libero, as they both change the type of the user based on the move it is about to use.
Because Pokemon Scarlet and Violet's Terastallization changes the base type of a Pokemon to its Tera type, having either Libero or Protean as its Ability might lead to it changing its type even after Terastallization. This could cause the game to miscalculate STAB damage either in favor of the user or its opponent, as a Pokemon with Protean or Libero always deals STAB damage, but its base type is being overridden by another mechanic.
Something similar can also happen when players try to actively change their opponent's type, like with a move called Soak. Soak makes the target drenched in water, and thus it becomes a pure Water-type until switched out or fainting. Its efficacy may depend on if the opponent Terastallizes prior to or after players use Soak on them, creating issues with the way the game determines the creature's type. Similarly, the move Forest's Curse adds Grass to the target's types, meaning that if a Pokemon hit by Forest's Curse Terastallizes as a Grass Pokemon, it will deal 2x damage with Grass moves compared to the usual 1.5x damage from STAB moves.
Changing a Pokemon's Type Can Cause Trouble Under Specific Conditions
Other problematic use cases for Pokemon Scarlet and Violet's Terastal phenomenon include double battles, where if a Pokemon changes its type it can have a huge impact on the course of the fight for both players involved. For example, a Pokemon with Storm Drain could become a sweeper if paired with a Volt Absorb Pokemon that Terastallizes into a Water type, as the latter could hit the former with its STAB Tera type move to boost its Special Attack, while also becoming immune to super-effective Electric damage.
Another way the Terastal phenomenon can become troublesome is if a trainer Terastallizes a currently disguised Zoroark that used its Illusion Ability upon entering battle. This is because Illusion transforms Zoroark into the last Pokemon in the user's party, which already makes it hard for the opponent to determine what sort of creature they are fighting. If Terastallization doesn't break the effect of Illusion, the opponent will have to guess if the base Pokemon is either Zoroark or the last Pokemon in the user's party, and then deal with the type change as well.
There are many instances where a battle gimmick that allows any Pokemon to change their type might be broken to some degree, and these are just some hypothetical scenarios. Still, it will be interesting to see how Pokemon Scarlet and Violet manage to balance its potential issues, and what combinations players can come up with when the games are released.
Pokemon Scarlet and Violet launch on November 18 for Nintendo Switch.