It is a GBA FireRed based Rom Hack with Gen 9 Pokemon, Paldean & Hisuian Forms, Paradox Pokemon & Much More!

Pokémon Legends: Arceus GBA is an unofficial, community-made ROM hack built on the 32-bit Game Boy Advance architecture (typically utilizing the Pokémon FireRed or Emerald base engine). It functions as a 2D retro "demake," aiming to adapt the lore, creatures, and visual style of the 3D Nintendo Switch title into an old-school handheld format.

World & Environment Design

• Open Map Layout: Rather than traditional, linear paths boxed in by ledges, the game heavily modifies the GBA tilemap to create wide, pseudo-open areas resembling a 2D interpretation of the Wild Area or Hisuian plains.
• The Hisui Aesthetic: Visual assets like battle backgrounds, overworld trees, and buildings are re-skinned using Black & White (Gen 5) style custom sprites to give the old GBA engine a fresh, historical wilderness atmosphere.

Battle Mechanics & UI Overhauls

• Redesigned HUD & Dark Mode: The battle screen features a sleek, custom user interface, frequently incorporating a "Dark Mode" summary page and modern selection bags.
• Hardcore Battle Constraints: To make up for the lack of real-time action gameplay, many versions lock the battle mode to "SET" and disable the use of healing items from your bag during active trainer matches to increase strategic difficulty.
• Survival Changes: Includes modern quality-of-life additions like the Black & White 2-style Repel system and "Poison Survival" (where your Pokémon will drop to 1 HP rather than fainting while walking in the overworld).

The Pokédex, Starters, & Custom Forms

• Backported Roster: The engine features an expanded Pokédex that introduces Generation 8 Hisuian variants (like Hisuian Zoroark and Growlithe) alongside hisuian evolutions like Kleavor, Wyrdeer, and Ursaluna.
• Bonus Cross-Gen Additions: Many common distribution builds bundle in Generation 9 Paldean Pokémon, custom Paradox variants, and updated attack data/learnsets matching Scarlet & Violet rules.
• Modified Starters: Rather than the classic Bulbasaur, Charmander, and Squirtle, the game replaces them with a Larpas, tailored to the unique theme of the hack.

Story & Structural Deviations

• While named after the Switch prequel, technical limitations mean the game often retains structural elements of classic GBA titles.
• Players begin the game encountering legacy characters like Professor Oak in a modified introductory sequence before branching into custom script events, rival battles, and an altered post-game questline dedicated to the mythical Arceus.

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