Pokémon Sword and Shield the Crown Tundra was announced during Nintendo Direct as part of the new Expansion Pass. It is set to release during Fall 2020, though it's the second one of the set.
Pokémon Sword and Shield the Crown Tundra: Everything You Need to Know
The Crown Tundra allows players to explore new parts of the Galar region, complete with new gyms, accessories, Gigantamax transformations and a greater pool of Pokémon. While Isle of Armor will have a more single player focus, the Crown Tundra will focus on exploration and will introduce a new co-op mode. The community also saw new legendaries and remodeled versions of Articuno, Zapdos and Moltres.
Nintendo is releasing a free update which will allow players to capture the new Pokémon set to release later this year. If you want to play either DLC, you need to spend $29.99 to pre-purchase the expansion pass.