How do you get the goblin army spawner on terraria on pe?

To obtain the Goblin Army Spawner in Terraria on mobile (PE), you need to defeat a Goblin Army event, which can be triggered by having at least 3 NPCs living in your world. Once the Goblin Army is defeated, you can find the Goblin Battle Standard in the loot. You can also craft a Goblin Battle Standard using 10 wood and 5 fallen stars at a workbench, which will summon the Goblin Army when used.