Special episodes of the Pocket Monsters anime distributed initially on Amazon Prime to promote the release of Pokémon LEGENDS: Arceus.
 
When Satoshi, Pikachu, and friends receive a mysterious message from the phantom Pokémon Arceus, they meet up with their old friend Takeshi and head to Mount Tengan to investigate. There, they find a rampaging Heatran and the group responsible—Team Galactic, previously thought to be dismantled. The team’s commanders are determined to find their missing leader by opening a gate between dimensions. With a trio of Legendary Pokémon and Sinnoh Champion Shirona on their side, our heroes have a lot of help, but they’ll need all they can get to save Sinnoh! 
(Source: Pokemon.com)