You will need a clean version of the Pokémon Emerald (U) ROM.

An Egglocke is a variation of the famous "Nuzlocke" challenge. While it retains the core rules—such as perma-death and naming every Pokémon—it adds a massive twist: every Pokémon you "catch" on a new route must be immediately swapped for a mysterious Egg. The Core Rules:

When searching for a "Pokémon Emerald Egglocke ROM download," it is important to understand that an Egglocke isn't usually a single .gba file you download. Instead, it is typically a combination of a standard and a Save File (.sav) containing the eggs. How to Set It Up: