Pokémon Emerald, but the Digimon of Digimon World 3 live in Hoenn.
When Melty sets it up, the mashup reads the Digimon's pixel-art sprites from your own Digimon World 3 disc and writes them, with new names, types, stats and moves, into a copy of your own Pokémon Emerald. Nothing from either game is included: you need both.
What you get (version 0.1)
10 Digimon with their DW3 sprites (front and back in battle, plus a menu icon): Agumon, Patamon, MagnaAngemon, Terriermon, Togemon, Renamon, Wizardmon, Piximon, Numemon and Megadramon.
They take the place of early Hoenn Pokémon, so you meet them right away: in the grass of the first routes and Petalburg Woods, in Team Aqua and Magma's teams, and even as Wally's first catch.
Each Digimon has its own types, abilities and moves inspired by its techniques: Agumon's Pepper Breath (Ember), Togemon's Needle Spray (Pin Missile), Wizardmon's Thunder Ball (Thunder Shock), Numemon's... Nume-Sludge.
The rest of Emerald is unchanged. Save files are kept in %LOCALAPPDATA%\DigimonEmerald and survive updates.
Single player. It plays in the bundled mGBA emulator. Keyboard: arrows to move, X = A, Z = B, Enter = Start, Backspace = Select. Controllers work too.
Games needed
Pokémon Emerald (US) as a .gba file of your own.
Digimon World 3 (US, SLUS-01436) as a disc image of your own (.bin or .iso).
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