Old One's Army

This is the one fight in Terraria where you are not the target. You place an Eternia Crystal on its stand, portals open on both sides, and everything that comes through walks toward the crystal rather than toward you. If the crystal dies the event ends on the spot, however well you were doing. Holding one position is correct here in a way it almost never is elsewhere, and covering the two lanes matters more than clearing the field.

Which version you get depends purely on what you have already killed. Before any mechanical boss it is five waves closing with one or more Dark Mages, worth five Defender Medals. After a mechanical boss it becomes seven waves closing with Ogres, worth twenty five. After Golem it is seven waves with mages and ogres appearing earlier and Betsy at the end, worth a hundred.

There is no menu for it. Carrying a mechanical boss summon into a pre-hardmode world and killing it there still leaves you on the first tier. The Tavernkeep unlocks stronger stock at each step, so farm the tier you are on until you can equip for the next one.

The crystal refuses to be placed if the ground is wrong, and the message does not say which rule you broke. It checks for:

• Sixty tiles of clear space on each side, measured from the stand. Give it more and the portals push out as far as 120 tiles each way.
• Ten tiles of vertical clearance between floor and ceiling.
• No hole in the floor wider than a single block.
• Nothing built directly against the sides of the stand.

You may build up to three tiles above the floor line before it counts as an obstruction, and platforms and inactive blocks are not floor, so they eat that allowance instead. Closed trap doors do count as flat and can be opened once the fight starts. Solid blocks on Actuators count as flat at placement, which is the trick for a floor you can reconfigure mid-event.

For the whole event a debuff blocks placing and mining blocks and stops buckets moving liquid. Everything has to exist before you activate the crystal, including anything you meant to add between waves. There is a thirty second countdown after each wave and hitting the stand skips it, but it is not building time. Things that do not work:

• Walling them out. They enter in a ghost state and return to it whenever no clear path to you exists, so blocks delay them and never stop them.
• Lava, water and honey. None of the three damages or slows an event enemy.
• Teleporters. Enemies ignore them, though you can still use them yourself.
• Traps as your main damage. They work, but the invincibility frames they cause block your sentries from hitting the same target, so a trap-heavy arena can output less than an empty one.

Etherian Mana is the resource the event runs on. It only drops here, it vanishes the moment the event ends whether it is on the floor or in your inventory, and it is the only fuel Tavernkeep sentries accept during the fight. You start with ten, which places one sentry, and every ten you collect places another. Those stay up until the event ends rather than timing out.

Sentries from elsewhere, a Queen Spider Staff or a Houndius Shootius, follow normal rules and sit alongside them, so bring both. Tavernkeep sentries placed with ordinary mana beforehand vanish when the event starts. Conveyor belts still work and drag loose mana to you, and mana in an unopened chest survives to the next attempt.

The Tavernkeep sells four sentry lines, Ballista, Flameburst, Lightning Aura and Explosive Trap, each in three escalating versions, plus armor per class per tier and the Defender's Forge.

Sentry first, armor second. The sentry is what lets you clear the tier at all; the armor mainly boosts whichever line it matches. Since a first tier clear pays only five medals, expect to repeat it before stepping up, and none of that is wasted because the gear works outside the event. The four sentry-boosting accessories are identical and do not stack, so buy one and pick on looks.

• In a hardmode classic world with no mechanical boss dead, a Blessed Apple or Ancient Horn mount runs down almost the whole first tier on contact. Only the Dark Mage needs attention.
• Minecart tracks are allowed and running enemies over works. An Ice Rod and Dirt Rod also keep working under the debuff.
• The crystal regenerates slowly when nothing is hitting it, and has twice the health on expert, three times on master.
• Each enemy counts as one kill toward a wave on classic, two on expert and above.
• No coins or biome keys drop here, so a Lucky Coin does nothing. Banners drop, but give a weaker buff than usual.
• The event is won the instant the final boss dies, even if you are dead and your sentries finished it. It is lost if the crystal broke first, even if you then kill the boss, though its loot still drops.