From Bulbapedia, the community-driven Pokémon encyclopedia.
A fateful encounter (Japanese: 運命的な出会い fateful encounter) is a flag attached to certain event Pokémon and some wild Mythical Pokémon that indicates they were met in a special manner.
In Generation III, it is used as an anti-cheat measure for Mew and Deoxys, penalizing players who obtained them through unofficial means. In Generation IV and V, starting with Pokémon Platinum, the presence of this flag has been used to trigger special events if the player has a certain Pokémon that was met in a "fateful encounter".
Pokémon obtained via event distributions often have the fateful encounter flag set. Events distributed prior to Pokémon FireRed and LeafGreen did not use this flag, as it was not used in Pokémon Ruby and Sapphire.
In Generation III, the fateful encounter flag is entirely invisible to the player, entirely separate to the "Fateful Encounter" met location. When migrated via Pal Park, it is the fateful encounter flag that is carried over, not the met location. While Pokémon can have both the flag and the met location, many Pokémon have the met location "Fateful Encounter" but not the flag, or have the flag but a different met location.
From Generation IV onward, "fateful encounter" is written alongside the met location to describe the encounter method.
Normally, only Pokémon obtained from event distributions can have the fateful encounter flag. However, there are some cases where it is possible to obtain Pokémon met in a fateful encounter in-game.
In Pokémon FireRed, LeafGreen, and Emerald, Mew and Deoxys that do not have the fateful encounter flag always disobey the player regardless of the player's Badges and cannot be traded away.
In Generation IV and V, fateful encounter Pokémon can trigger certain in-game events.
In Pokémon Sword and Shield, any Shiny Pokémon met in a fateful encounter will appear with square sparkles, regardless of its personality value.
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