The official Pokémon website has announced that Giovanni and Mewtwo Return to Pokémon Masters for a limited time from March 5, 2020, at 10:00 p.m. PST until March 15, 2020, at 9:59 p.m. PDT.

Giovanni & Mewtwo Return to Pokémon Masters

Revisit the Lurking Shadow legendary event, add this incredible sync pair to your team, and power them up more quickly than before.

The island of Pasio will be in trouble once again, as the 5★ sync pair Giovanni & Mewtwo are preparing to make a return appearance. The Lurking Shadow legendary event will return for a limited time from March 5, 2020, at 10:00 p.m. PST until March 15, 2020, at 9:59 p.m. PDT. As you complete this event, you'll earn items that you can use to add Giovanni & Mewtwo to your roster. With this sync pair on your side, you'll be able to take advantage of their powerful sync move, World Domination Psystrike, and their ability to use both Psychic-type and Ghost-type moves makes them a useful ally.

This time around, the Lurking Shadow event will include new single-player battles. There'll also be some rewards adjustments, so even if you played through this event the first time, it'll be worth revisiting. Changes will be made to the custom items required to strengthen Giovanni & Mewtwo, making it easier to power them up as well.

Source: https://www.pokemon.com/us/pokemon-news/giovanni-mewtwo-return-to-pokemon-masters/