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Tornadus (Sword) and Thundurus (Shield) are unlocked by speaking to Cheren, Bianca, and N. Bianca forces you to team up with Bede to battle Cheren and N. Speaking to N again with the two Legends unlocks Landorus. All three Legends will always be in their Therian Forms in Max Raid Battles and Incarnate Forms in the Wild Area. The Reveal Glass also does NOT return, so the forms they're in is permanent.
There are also benefits and downsides to battling them in Raids vs Wild Area. In Raids, each Legend will always have Hidden Abilities, but will be Shiny Locked, while in the Wild Area, they have normal abilities, but increased Shiny rates equal to the level of the Masuda Method.
The next two Legends are surprise returns that I wasn't expecting.
Type: Null returns and is given to you by Gladion after showing him six different Pokémon with max happiness. Speaking to him again after Type: Null evolves will net you the Memory items.
