Nebula

It's a Library for creating Spells in all sorts of ways. Nebula adds Spell, Mana adds Client Syncing.
If you consider Nebula or are stuck on anything feel free to reach out to me on my Discord Server.
If you want full control you can override the SpellManager or ManaManagers! With that you can basically change everything!
_{Only one Mod can override a Manager.}

You can include Nebula in your build.gradle:

repositories {
    exclusiveContent {
        forRepository {
            maven {
                name = "Modrinth"
                url = "https://api.modrinth.com/maven"
            }
        }
        filter {
            includeGroup "maven.modrinth"
        }
    }
}

dependencies {
    modImplementation "maven.modrinth:nebula:4.0.3"
}

Tutorial

You can see the Nebulo Test mod, it does implement a Spell and shows you how to override the Managers.

If you need any further help reach out to me on Discord

Old

public class SuicideSpell extends Spell {
    public SuicideSpell(SpellType<? extends Spell> spellType, PlayerEntity caster) {
        super(spellType, caster);
    }

    @Override
    public void cast() {
        getCaster().kill();
    }
}

Registering the Spell

        public static SpellType<SuicideSpell> SUICIDE = SpellType.register(new Identifier("yourmod", "suicide"), SpellType.Builder.create(SuicideSpell::new, 1));

We registered the Spell you need to replace "yourmod" with your modId. The Spell costs 1 Mana.