@SubscribeEvent
public void onVillagerTrades(VillagerTradesEvent event) {
        ItemStack spawnEgg = new ItemStack(Items.ZOMBIE_SPAWN_EGG);

        CompoundNBT nbt = new CompoundNBT();
        nbt.putBoolean("IsCursed", true);

        CompoundNBT forgeData = new CompoundNBT();
        forgeData.put("ForgeData", nbt);

        if (!spawnEgg.hasTag()) {
            spawnEgg.setTag(new CompoundNBT());
        }

        spawnEgg.getTag().put("EntityTag", forgeData);
        event.getTrades().clear();
        for (int i = 1 ; i <= 5 ; i++) {
            event.getTrades().put(i, Arrays.asList(new BasicTrade(new ItemStack(CustomItems.GOLD_COIN.get(), 10), spawnEgg, 1, 100, 1)));
        }
    }

    @SubscribeEvent
    public void onEntitySpawn(LivingSpawnEvent.SpecialSpawn event) {
        if (event.getSpawnReason() != SpawnReason.SPAWN_EGG) return;
        LivingEntity entity = event.getEntityLiving();
        System.out.println(entity.getPersistentData());
        if (entity.getPersistentData().getBoolean("IsCursed")) {
            System.out.println("Entity is cursed!");
        }
    }