How to make mobs be a 'pet' to another mob

So basically I want to make a necromancer mob that can control undead and make them attack. If they get attacked by the player all the undead around it should attack the player afterwards.

Wrote this up just for you😘 lol;

    @SubscribeEvent
    public void necromancerDamage(LivingDamageEvent event) {
        if (event.getEntity() instanceof NecromancerEntity && event.getSource().getTrueSource() instanceof LivingEntity)
        {
            List<MobEntity> mobEntities = event.getEntity().world.getEntitiesWithinAABB(MobEntity.class, event.getEntity().getBoundingBox().grow(10));
            Iterator iterator = mobEntities.iterator();
            while (iterator.hasNext()) {
                MobEntity mobEntity = (MobEntity) event.getEntity();
                if (mobEntity.isEntityUndead())
                {
                    mobEntity.setAttackTarget((LivingEntity) event.getSource().getTrueSource());
                }
            }
        }
    }

 also want the necromancer to drop it's robes, which let the player control the undead.

Explain more, like how control them. Control movement, etc.

Ok, something similar to this should work, I am on my phone right now so some of the methods may be incorrect.

public void livingAttackTarget(LivingSetAttackTargetEvent event) {
if (event.getTarget().inventory.getItemStackInSlot(EquipmentSlot.CHEST) == new ItemStack(YOURNECROMANCERROBES) && event.getEntity() instanceof MobEntity) {
((MobEntity)event.getEntity()).setAttackTarget(null);
((MobEntity)event.getEntity()).setAggroed(false);
}
}

Since these are events, they can be in any registered class using,

MinecraftForge.EVENT_BUS.register(TheClass);

Post what you have

Don't use the code I provided with the procedure wrapper. It provides dependencies, not the event.

Instead you should create a custom element, register it, and then use the code in its entirety.

Can you post the error?