Although you get Koraidon or Miraidon at the beginning of Pokémon Scarlet and Violet, you are not actually able to battle with them for most of the game. Here is what you need to know to unlock the powerful Battle Forms for the Gen 9 Legendary Pokémon.
How do you unlock Koraidon and Miraidon Battle Forms in Pokémon Scarlet & Violet?
The Battle Forms for Koraidon and Miraidon will only unlock once you have beaten the game. After completing the story campaigns Victory Road, Starfall Street, and Path of Legends, you will unlock a hidden 4th campaign that takes you to the Great Crater of Paldea. Once you have finished the main storyline, you will finally unlock the Apex Build for the Scarlet and Violet Legendary Pokémon, which allows you to use them in battle.
How do you use Koraidon and Miraidon in battle in Pokémon Scarlet & Violet?
After unlocking their Battle Forms, you will now be able to use Koraidon or Miraidon whenever you want in battle. Both Legendaries start at level 68, so you might need to train them up a bit if you plan on taking on 7-star Tera Raids. Because Koraidon and Miraidon are also your vehicles, you will first need to make them battle ready with a method that is described below.
How to switch Koraidon and Miraidon to their Battle Apex Build in Pokémon Scarlet & Violet
In order to battle with Koraidon or Miraidon there first needs to be an open slot on your Pokémon team. Now that you have an empty slot, press the X button to open up your team menu and move your cursor down to Koraidon or Miraidon. Next, press the A button on the Legendary to pull up a prompt to change it into its Battle Form. Select yes, and the dragon will now be moved to your team in Battle Form. To change the Pokémon into your vehicle again, simply press the plus button which will automatically transfer the Legendaries back to their Riding Forms.
For more information on Pokémon Scarlet and Violet, check out Where to find Eevee and its evolution stones here on Pro Game Guides.