If you want to create a version where Gym Leaders have six Pokemon or where you can catch all 721 species without trading, you start with a decrypted ROM.
Using a Mega Stone, certain Pokémon can reach a powerful temporary form mid-battle. Pokemon Y 3ds Rom Decrypted
When you dump a game directly from a physical 3DS cartridge or download it from the eShop, the file is usually encrypted. This encryption is a security measure designed by Nintendo to ensure the game only runs on official hardware. If you want to create a version where
A decrypted ROM cannot be played on a stock 3DS. However, you can use a (like the Sky3DS+) that accepts .3ds files. Note that many flashcarts require encrypted ROMs; you would need to re-encrypt your decrypted ROM using a tool like simple_3ds_converter . This is rarely worth the effort. This encryption is a security measure designed by