Initially revealed on a recent episode of the Japanese show Pokenchi, Serebii.net has since broken down all the new details.
Featuring up to four players, the primary goal will be to reach the end of the den were players will be met by a legendary Pokémon. Prior to this, they’ll need to defeat three Dynamax Pokémon, and any HP or PP that your rental Pokémon lose will carry between battles unless you a character that will heal you. Alternatively, you can also find berries that will replenish HP and PP.
Defeating the legendary Pokémon at the end of the den will offer up the opportunity to catch it. Every legendary Pokémon (aside from mythicals) will be available to catch, although it looks like we’ll only be able to catch one of each. It’s unclear as to what will happen if you encounter the same legendary (if that is even possible).
Dynamax Adventures are just one new feature coming with The Crown Tundra expansion. Others include the Galarian Star Tournament and another wave of Pokémon.