You've probably seen the Tavernkeep wandering around your world after you’ve beaten the Eye of Cthulhu or the Eater of Worlds. He looks a bit out of place. That’s because he is. He’s a direct transplant from Dungeon Defenders 2, and he brings a specific brand of chaos called the Old One's Army Terraria event. Most players treat this like a standard invasion. They think they can just swing a Zenith or a Terra Blade and call it a day. That is exactly how you lose your Eternia Crystal in under three minutes.
This isn't a normal fight. It’s a tower defense game shoved into a 2D sandbox.
The mechanics are weird. Honestly, they’re frustrating if you don’t understand how "Sentry" power works here. You can’t just spam your usual summons. You need Defender Medals. You need specific turrets. And you need to realize that the enemies in this event have a "ghost" property that lets them pass through solid blocks if they get stuck. You can't just cheese this with a lava pit and a box.
How the Old One's Army Scales With Your Progress
The game checks your boss progression before it decides how hard to hit you. There are three distinct tiers. If you try to start the event for the first time after killing Golem, you’re going to get absolutely rocked by Tier 3 difficulty without having any of the Tier 1 or 2 gear to back you up.
Tier 1 happens in Pre-Hardmode. It’s five waves of basic fodder like Etherian Goblins and those annoying Flying Kobolds. It’s manageable. Tier 2 kicks in after you’ve downed one Mechanical Boss. It jumps to seven waves and introduces the Ogre. He snot-spits at you. It’s gross, and it slows you down, making you a sitting duck for the Drake's fire breath. Tier 3 is the real nightmare. This unlocks after Golem is defeated. This is where Betsy shows up. She’s a dragon that behaves a lot like Duke Fishron but with more AOE fire.
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If you haven't farmed the Medals in the earlier tiers, you'll be stuck using basic weapons against a Tier 3 horde. It’s a recipe for a quick "The Eternia Crystal has been destroyed!" message.
The Setup That Actually Works
You need flat ground. Not "kinda flat." I mean perfectly flat.
The Eternia Crystal Stand requires a long, unobstructed stretch of land. If the ground is too uneven or too short, the game simply won't let you start the event. You need at least 61 tiles of flat space on each side of the stand. Ideally, make it 100 tiles. Don't use platforms; use solid blocks like Hay or Stone. Why? Because you want to be able to run at full speed with Hermes Boots or their upgrades without accidentally dropping through a floor.
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Why Sentries Are Non-Negotiable
In the Old One's Army Terraria event, your standard mana and minion slots don't work the same way. You use Etherian Manna. This is a special resource that drops from enemies during the waves.
- Flameburst Cane: Great for concentrated fire.
- Ballista Cane: High knockback, slow fire.
- Lightning Aura Cane: Ignores defense. This is the secret sauce for groups.
- Explosive Trap Cane: Basically a proximity mine.
Don't sleep on the Lightning Aura. Most people think the Ballista is better because the numbers are bigger. They’re wrong. The Lightning Aura creates a field of constant damage. In the later waves, when the screen is literally filled with Goblins and Drakin, that constant tick-damage is the only thing keeping the Crystal alive.
Betsy: The Boss Most People Forget
Betsy is the final boss of Tier 3. She doesn't just fly; she strafes. She breathes fire that covers a massive portion of the arena. If you’re playing on Expert or Master Mode, her dash attack can one-shot players who aren't built for tanking.
You have to treat her like a bullet hell boss.
Focus on her, but don't ignore the ground troops. This is the hardest part of the Old One's Army Terraria event. You have to split your attention. If you spend 30 seconds chasing Betsy across the sky, a stray Kobold Glider might just smash the Crystal while you’re gone. This is where the Red Riding or Shinobi Infiltrator armor sets come in. These are sold by the Tavernkeep and specifically buff the sentries you’re using.
Using the "Valhalla Knight" set is a popular "pro" move. The health regeneration on that armor is so high it’s basically broken. You can stand in the middle of a swarm and just... not die. It’s one of the few ways to solo the Tier 3 event without losing your mind.
Common Mistakes That Kill Your Run
Stop building walls. I’ve seen so many players try to build a "fort" around the Crystal. The Etherian enemies have a specific AI trigger. If they can’t find a path to the Crystal, they go into a "ghost" state. They turn translucent and float through every block you’ve placed. By building walls, you’re actually making the game harder because you can’t hit them as easily while they’re phasing through your base.
Also, watch out for the Etherian Wyverns. They fly high and dive down. If your turrets are all on the ground, the Wyverns will chip away at the Crystal's health from above. You need to place at least one or two sentries on elevated platforms specifically to catch the aerial units.
Loot Worth the Grind
Why do this? Why subject yourself to seven waves of misery?
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The loot is actually incredible. The Flying Dragon sword shoots projectiles that pass through walls. The Aerial Bane bow is arguably one of the best wings-user weapons in the game. And then there’s the Betsy’s Wrath spell. It applies a debuff called "Betsy's Curse" which reduces enemy defense by 40. That is huge. It’s one of the most powerful support items for late-game boss fights like Moon Lord.
Actionable Steps for Success
- Clear the Land: Build a 200-tile flat bridge. Use a Ruler to make sure it’s even.
- Farm Medals Early: Don't wait until Hardmode. Do the Tier 1 event 5–10 times. You want to buy the Flameburst or Lightning Aura rod before the difficulty spikes.
- Use the Tavernkeep’s Gear: His armor sets are hybrid. They buff your own damage and your sentries simultaneously. This is the only way to have enough "firepower" to cover both sides of the arena.
- Prioritize the Ogre: In Tier 2, the Ogre is the threat. His jump attack creates a shockwave. Jump when he jumps.
- Ignore Mana, Use Etherian Manna: Your blue stars don't matter for the turrets. Collect the green orbs dropped by mobs and immediately spend them. Never finish a wave with unspent Manna in your inventory.
The event ends when the Crystal is safe or Betsy dies. If you're struggling, bring a friend. The event doesn't scale health based on player count as aggressively as other bosses, making it much easier to manage the two-front war with a second person covering the left side.