https://www.gamespot.com/articles/a-fan-favorite-pokemon-go-legendary-is-returning-f/1100-6457438/

One of Pokemon Go's first Legendary Pokemon will make another appearance in the mobile game very soon. Beginning March 16, Pokemon Go players will have another opportunity to capture the Gold and Silver Legendary, Lugia, for a limited time.

As before, players will encounter Lugia as a Raid Battle boss at Gyms. This time around, however, Pokemon Go developer Niantic says the Legendary Pokemon will know the "newly improved" Flying move Sky Attack, although Niantic didn't elaborate as to how the attack has been improved.