The Legendary Lugia will return to Pokémon GO Raid Battles with the move Sky Attack from tomorrow, March 16, until April 2. The official announcement from the development team at Niantic is as follows:
The Legendary Pokémon Lugia will return to Raid Battles in Pokémon GO on March 16! A powerful Psychic- and Flying-type Pokémon originally discovered in the Johto region, Lugia will appear in Gyms across the globe as the Legendary Pokémon Rayquaza returns to the ozone layer. You can challenge Lugia, which will know the newly improved Flying-type move Sky Attack, until April 2. So gather your strongest party to take on this Legendary Pokémon before it returns to the sea!
—The Pokémon GO team
Source: Official Pokémon GO blog