Goblin Punter

The Goblin Army invades on its own once you've smashed a Shadow Orb/Crimson Heart (and have 200+ HP), or you can trigger it by using a Goblin Battle Standard (10 Spiky Balls the goblins drop + tattered cloth). Fight it at your base with a bit of choke-point architecture and lava moats if you're fancy. Beating it frees the Goblin Tinkerer to be found underground - reforging and the Tinkerer's Workshop are the real prize.