Custom Showdown Datapacking

Abilities

To add custom Pokémon abilities in the game data/namespace/mega_showdown/showdown/abilities/ability_name.js

{
    onSwitchIn(pokemon) {
      this.effectState.switchingIn = true;
    },
    onStart(pokemon) {
      if (this.effectState.switchingIn) {
        this.add("-ability", pokemon, "Air Lock");
        this.effectState.switchingIn = false;
      }
      this.eachEvent("WeatherChange", this.effect);
    },
    onEnd(pokemon) {
      this.eachEvent("WeatherChange", this.effect);
    },
    suppressWeather: true,
    flags: {},
    name: "Ability Name",
    rating: 1.5,
    num: 76
}

This wiki wont exactly teach you how to make abilities and showdown logic, you can refer other existing abilities to craft your custom one from Showdown

Conditions

HeldItems

Moves

Scripts

TypeCharts