I want to make a item, which when a entity is attacked with it, it gives that entity a potion effect.

 

Any help would be greatly appreciated, Oscar

override onLeftClickEntity and addPotionEffect to the living entity

@Override
    public boolean hitEntity(ItemStack stack, LivingEntity target, LivingEntity attacker) {
        target.addPotionEffect(new EffectInstance(Effects.WEAKNESS, 200, 0));
        return super.hitEntity(stack, target, attacker);
    }

I can't get that to work, IntelliJ keeps on saying that it can't override the superclass method. Do you know how to fix this?

It's fine, I have managed to get what I intended to work with PlayerInteractEvent instead.