Guide:Old One's Army strategies

The Old One's Army event differs from others in that the invading enemies do not attempt to kill the player; instead, they focus on destroying the Eternia Crystal used to summon it.

The invasion is separated into three Tiers depending on progression. It will start at Tier 1, and after a Mechanical Boss has been defeated Tier 2 can be fought. Then, after taking down Golem for the first time, Tier 3 is available.

The objective is to protect the Eternia Crystal from the enemies until all waves have been defeated.

Summoning Conditions
In order to summon the event, the Player must place the in a roughly flat terrain: a maximum of three tiles of vertical deviation is allowed, any more and the Stand will not accept the.

The arena needs to be at least 61 tiles long on each side of the Eternia Crystal Stand, measured from the center of the Stand (a 123-tile clearing in total, with the Stand at the center tile). The enemy portals will spawn further away if given the space however, up to 120 tiles away from the center of the Stand on each side. The area also needs 10 tiles of clear vertical space.

Arena
Before facing the event, making a completely flat arena works very well. If an enemy's path to the Eternia Crystal is blocked, it will phase through obstacles, so trying to stop their progress with blocks won't be very effective. Building the floor with will greatly increase movement speed. Using Potions is always a good idea, while it is also good to place, , and  to survive for longer. When fighting Tier 2 and 3, building a ceiling above the Eternia Crystal will protect it from Kobold Gliders, since they explode on contact with blocks.

Build a raised platform 20 blocks off the ground, halfway between the Eternia Crystal and the expected portal location. Placing Flameburst Towers on these platforms will allow them to target flying enemies more easily, which can reduce swarming of the Crystal in later waves.

Sentries
Using one of the Tavernkeep's Sentries is also highly recommended, as they will greatly help when dealing with the many enemies the player will encounter. There are four different types with three tiers: Rod, Cane, and Staff, with the last being the strongest.


 * have average damage, fire speed and range. They shoot fireballs which deal splash damage and inflict the debuff. When using  or, their range is greatly increased.


 * have the highest damage, but also the slowest fire speed. They shoot bolts that can pierce through enemies. When wearing or, the bolts will pierce more and the player will get the  buff when hit, which will double their fire speed. In addition, the Valhalla Knight armor will increase the attack speed.


 * are placed on the ground, and they will explode when enemies get near it. They do not disappear after exploding. When used with or, they will attack much faster and  oil enemies, making the , , , and  debuffs do 2 more damage over time.


 * are a circle of lightning centered on the base that damages enemies repeatedly as they walk through them. They have the lowest base damage, but their damage is unaffected by enemy Defense. When used while wearing or, they will deal damage slightly faster and they will receive a set 16% (for Monk) or 25% (for Shinobi Infiltrator) critical strike chance. In addition, the Shinobi Infiltrator set will increase the area of effect.

After the event is summoned, the player will obtain ten, enough to summon one Tavernkeep's Sentry. Two Mysterious Portals will appear on the opposite ends of the arena; this where the enemies will spawn. Enemies will drop Etherian Mana when killed, allowing a player to summon more sentries as the event goes on.

It is easy to get overwhelmed when fighting the enemies coming from both sides, so the player should constantly move to the left and right of the Crystal in order to prevent the enemies from reaching it. They should also always be on the watch for Etherian Wyverns and Kobold Gliders, as they can quickly get to the Crystal and deal a lot of damage to it. The Tavernkeep's Sentries will deal with the weaker grounded enemies; however, they are not very effective against flying enemies.

It isn't necessary to get every sentry for each tier. An effective strategy is to use a close-range sentry, like the Lightning Aura or Explosive Trap at the portals and a long-range sentry like the Ballista or Flameburst closer to the center.

Boulder Drop
An effective and easy to use strategy, especially pre-hardmode. First, you must add a roof to your arena, about 15-25 blocks above the ground. Then, you must collect as much stone as you think is necessary, which may take quite a bit of time. You must then take a Heavy Workbench to a graveyard and craft as many Boulder Statues as possible. Bring the boulder statues back to your arena, and place them along your newly constructed roof. You should get some wire, wrenches, and a timer, while a switch is recommended, all of which can be bought from the Mechanic. All you need to do now is connect the wires to the Boulder Traps and timer, and place two blocks on each side of the Eternia Crystal to prevent the boulders from damaging it or you, without being too high to start the event. You may also place some platforms above the boulders, so you can still move around without taking damage from them. Note that the boulders only fall every 15 seconds, which means you should leave enough space for the portals to spawn at maximum distance. You will also want the ground to be flat, so the boulders can go all the way across the arena. Using this tactic, you only need to defeat flying enemies, such as Etherian Wyverns or Lightning Bugs, along with any bosses that spawn. Note that this tactic works best in lower tiers, and only adds additional damage to enemies on higher ones.

Actuator Drop
Since the crystal stand can be actuated, the entire event crystal can be dropped down, much like the flat terrain the event requires. If the stands on sufficient ground for the various grounded enemies in the event to stand on, or on its initial elevation, they will float toward the crystal with a shadow aura and still reach it. However, if the crystal is dropped to a single platform or block, and grounded enemies are dropped beforehand (recommended roughly 40 blocks from the center), they'll only float if they get far enough down from the crystal, in which case they float diagonally toward the crystal. When employing this strategy, you should consider placing a water platform (using ) and using some iteration of or a  to provide yourself with a platform underneath the crystal, but also to cancel enemies floating toward the crystal: since water is considered a physical block, they'll stop floating. The by far most effective strategy, to solve the issue of floating, is on the other hand placing a long platform of blocks at least 65 blocks below the crystal entity's platform, with a 5 wide by 10 high chamber in the middle. If the is dropped down onto a single block, the enemies are dropped down 40 blocks away from the center and a platform 65 blocks down is built, with a 5 by 10 chamber, these are the results:


 * No ground based enemies, i. e. no, , , , or  are able to be in range of or reach the crystal, and effectively deal no damage, with one exception, discussed below.
 * A can be placed in the 5 by 10 chamber to provide consistent damage against all of the above. The tougher enemies (not  or ) won't consistently spend time in the chamber (instead they'll skip out) but since they can't damage the crystal this is irrelevant to the progress of the event. Using higher level  provides faster progression, which is useful when you're trying to farm the event for.
 * Note that this is a much better solution than only using summons, since only using summons can actually incite to float up toward the crystal. Using summons in conjunction with a  is effective.
 * An alternative is not to use but instead rely on piercing weapons such as the . This strategy is extremely viable since the NPC limit of 200 is quickly reached when the player isn't actively killing the enemies, which means that the flying enemies attacking the portal will spawn predictably and in lower numbers. The number of enemies spawning from new waves will also be controllable.The main downside to this is that  can follow you and float toward the crystal if inadequately dealt with when visiting the platform. Standing right below the crystal does temporarily disable their floating when they reach your position, but due to their high damage this can be extremely deadly, especially considering the high health of.
 * The and  are unable to damage the . The 40 blocks in distance from the center are valuable since the Dark Mage will stop roughly that length away from the crystal and try to summon, a spell which will fail since there is no ground for them to be summoned on. The Dark Mage is thereby unable to inflict any damage. Likewise, the 65 blocks down the platform was placed is specifically such that the Ogre's balls of snot won't hit a player defending the crystal slightly from below, or the crystal itself.
 * Effectively, the player can go the all three tiers worrying solely about, and , as well as . For enemies, placing  is an effective way to provide damage and knockback high enough to keep the flying enemies away from the crystal, even more so with summoner gear. With effective placement and sufficient armor, the event becomes mostly automatic even in , apart from . It's always a good idea to keep an eye on the  at all times, on the other hand, especially in earlier waves.

Tips
Do note that fly in line with the crystal's height rather than quite far above it like the other flying enemies, so anyone entering Tier 3 should be ready to place   close to and in level with the crystal to ensure aid with the bugs.

Additionally, make sure to bring a and a  to be able to safely teleport under the platform and collect the. Also, consider building the platform out of to collect the mana in a more central location.

Most importantly, to be able to view the while killing the, collecting , or personally killing the enemies in the box, bring , a  or simply use any iteration of the  to provide an increased range of view and some oversight of the crystal. Ranged builds are recommended for this strategy since they can utilize a scope for accessible good sight and high precision in killing the flying enemies. Use non-homing bullets.

Consider placing banners for, and  if the arena is placed relatively high above the surface, since they can spawn during the event and be a significant distraction, as well as deal damage. Especially.

Tier 1
Tier 1 is the easiest, as it possesses the weakest enemies and lasts five waves. It can only be summoned after the Eater of Worlds/Brain of Cthulhu have been defeated, as then the Tavernkeep will spawn for the first time.

The player will see five different enemies:
 * The is the basic enemy of the event as well as the most common. It doesn't possess high stats or a special attack, making them very easy to kill.
 * The and  are stronger enemies with ranged attacks. They will start spawning on Waves 2 and 3 respectively.
 * is a highly damaging enemy with the ability to fly. It will quickly fly towards the Crystal, and then it will dive towards it. It will start spawning on Wave 4.
 * The  is the Boss of the tier, spawning on Wave 5. It will alternate between shooting magical energy at the player, summoning  and healing all enemies around it. It needs to be killed in order to finish the event.


 * Ballistas are not fast enough to kill the grounded mobs, but three Lightning Aura sentries at each entrance can kill them quickly enough. In this mode, Etherian Wyverns can destroy your crystal in just seconds, so placing a couple Ballista sentries on a platform about 20 blocks off the ground can help in killing them.

Recommended gear
While the event can be summoned at any time after defeating Eater of Worlds/Brain of Cthulhu, it's better to use the best gear available in Pre-Hardmode.

Armor

 * provides the highest defense and good bonuses for Melee characters.
 * will give greater bonuses to Ranged characters.
 * The Wizard set with and  or  provides the best offensive  bonuses.
 * A Summoner should not attempt the event without obtaining first.
 * A Thrower should get from the Desert. If you don't want to spend your time getting it, the  is a good, but still weaker, alternative.

Weapons
In addition to their normal weapons, all classes should also use one of the Tavernkeep's Sentries.


 * For Melee users:
 * The, or the  are great weapons at close range.
 * If obtained, the or the  can do great damage to all enemies despite its short reach.
 * The is a great weapon, but it's better suited for single targets.
 * The is great against crowds.


 * For Ranged users:
 * The with  is the best gun available.
 * A with  will excel against crowds. The  can serve the same purpose.
 * The is also a very strong weapon to use.
 * For Magic users:
 * The and  excel against crowds. Water Bolt travels back through the enemies if a wall is placed behind each portal beforehand.
 * The and  are better against single targets.
 * The is an acceptable secondary weapon.
 * The 's infinite piercing clears most enemies in this event.


 * For Summoners:
 * At this point in the game, the best minions available are the and.


 * For Throwing users:
 * ,  and  are the best throwing weapons to use for crowd control.
 * The is also an effective weapon.
 * are best for single targets.

Accessories

 * Mobility accessories such as the, variants of the and  are always good to have.
 * The provides a high damage upgrade to all weapons. The  is also good when using Bee weapons.
 * are useful to quickly regain mana as a mage.
 * is better as it provides steady mana regeneration when not using magic weapons.
 * can reduce damage taken by 17%.
 * In Hardmode, the Ancient Horn (Basilisk) or Blessed Apple (Unicorn) allows quick movement and charging through enemies.

Tier 2
After a Mechanical Boss has been defeated, the second tier can be fought.

The player will find five new enemies, and the four basic ones from the previous tier will get significantly higher stats, which is reflected on their different designs.
 * are one of the new enemies found. When near the player or the Eternia Crystal, it will quickly dash towards them, exploding on contact. The explosion deals high splash damage.
 * In the later waves, the player will also find, a variant with much lower stats but the ability to fly like Etherian Wyverns.
 * have high health, defense, and knockback immunity, making them very hard to kill. They are very slow, however they will fire magical shots from a distance.
 * will surround themselves with an aura of crystals which will inflict the debuff, halving defense of players inside of it. They also possess high health, and they can regenerate damage taken.
 * The is the boss of the tier. It possesses very high damage and health, and does three dangerous attacks. From a long range, the Ogre will hurl a ball of snot at a target, it will only damage targets it hits but it will inflict the  debuff on any nearby player, slowing them down significantly. It can also jump into the air and land with a shockwave, doing area damage about ten blocks forward in each direction on the ground. At very close range, it will strike with its club, dealing very high damage. Like the Dark Mage, it needs to be the last enemy killed in order to finish the event. When killed also drops an unique Accessory or Weapon.

Armor

 * is a strong choice, in the 1.4 update with the Holy Protection buff.
 * is very good for all classes except Summoner.
 * , is a great choice due to its moderately high stats and the buff. Though with the 1.4 update the set bonus buff had been moved to Hallowed Armor, and replaced with Titanium Barrier
 * is especially useful as its set bonus can hit many enemies at once.
 * may be useful due to its set bonus.
 * is the best choice for summoners pre-Plantera, and the is the best post-Plantera. Both will boost the strength of all of your sentries, making them especially strong choices.
 * should only be used if the player wants to mix summon and magic weapons, as it doesn't provide enough stat bonuses for either individual class to be useful.

Weapons
Just like in Tier one, all classes should also use one of the Tavernkeep's Sentries with their normal weapons.


 * For Melee users:
 * The and the  are the strongest swords available pre-Golem.
 * With enough attack speed bonuses, the is an extremely powerful weapon.
 * is a very good weapon as well.
 * The deals massive damage to crowds of enemies.
 * The and  are the strongest yoyos available.
 * The and  are both very strong Spears.
 * The is good against both crowds and single targets.
 * A or  should always be used if available.


 * For Ranged users:
 * The and  with  are the strongest bows available.
 * The  performs extremely well against the lines of enemies that will be faced in the event.
 * The with  is very strong against single targets.
 * The or  with  or  excel against crowds, while  are better for single targets.
 * It is always good to have a secondary weapon with ammo.
 * For Magic users:
 * The is excellent against crowds. The  is useful as well.
 * The is good for all enemies.
 * and are very strong against single targets.
 * The and  are very good secondary weapons.
 * The can be used effectively as its high knockback repels enemies.


 * For Summoners:
 * The, , , , , , ,  and a handful of whips are available.
 * The is very powerful but can be very hard to get.
 * and can be used as an extra sentry.

Accessories

 * are extremely useful in hardmode.
 * Mobility accessories such as the or  are very useful as well.
 * Damage increasing accessories such as any Emblem, the or  and  are always a good choice.
 * Variants of the and  can prevent hits.
 * are still good for regaining Mana.
 * is effective against flying enemies if you stand at the center against a backdrop of walls. is effective against grounded enemies that get close to you.

Tier 3
After the Golem has been defeated, the third and final tier of the Old One's Army can be fought.

Like the previous tiers, enemies will get significantly higher stats, and some will also get new designs. There are also two new ones:
 * will quickly fly near the Crystal, and attack it with energy projectiles. They will inflict on hit, which will temporarily halve the damage output of all weapons.
 * Betsy, who will spawn on Wave 7 serving as the final boss of the event. Unlike the other enemies, she will directly focus on killing the player instead of the Crystal. Her attacks consist of spinning in circles before charging multiple times, breathing fire, shooting fireballs and spawning Etherian Wyverns. While her attacks don't deal too much damage to the Crystal, the threat of the other enemies can make the final wave significantly harder than the others. Like with the Dark Mage and Ogre on Tiers 1 and 2, killing her will end the event.

Armor

 * is the best choice for a Melee character. The Mail gives significant buffs to offensive stats, while the Shell adds a great amount of damage reduction.
 * is the best for a Ranged user. Its set bonus is particularly useful since the player can beat the event by shooting the enemies while standing in the center.
 * is the best for a Mage. The Hood should only be used when healing is needed, since it will greatly reduce damage dealt.
 * Summoners should summon their minions and sentries while wearing, and then swap to for an extra minion. Note that this no longer works as of , since all summon damage is constantly recalculated.
 * All classes can benefit from the tier 1 armors sold by the Tavernkeep. While they are less powerful than the above armors, they boost sentry power significantly, allowing the player to focus on Betsy without worrying about the rest of the Old One's Army. The  upgraded versions of their armor are even more powerful, and can be purchased with Defender Medals earned from beating the Tier 2 Old One's Army or from beating rounds 1-6 of the Tier 3 Army.

Weapons

 * For Melee users:
 * The and the are the strongest swords available.  and the  are effective as well.
 * The is a great spear for crowd control as its projectiles cover an very wide area. It is very effective on Wave 7, since it can handle Betsy and the enemies at once.
 * With the right setup, the can deal an extreme amount of damage to all enemies and bosses.
 * is the strongest yo-yo available.
 * The and  can beat the event, although there are better weapons available.


 * For Ranged users:
 * The with  is the strongest bow available. The  with  is great against crowds, however it is very weak against Betsy.
 * The and  or Chlorophyte Bullet is an extremely strong gun. The  is a noticeably weaker alternative.
 * The is very powerful during the entire event, as it is great against enemies and bosses alike. The  is an alternative.


 * For Magic users


 * The is capable of firing from one portal to the other, piercing every enemy along the way, and killing all but bosses instantly
 * is a very effective weapon against enemies while still being useful against the bosses.
 * The and  both have extremely high DPS. The  is a weaker alternative.
 * The, , and are great secondary weapons. The  is still useful against bosses.


 * For Summoners


 * The is the best minion available. As of update 1.4 on PC, both  and  are very effective, as well.
 * The can be used as an extra sentry that doesn't cost Etherian Mana.
 * makes your minions do additional damage to nearby enemies, letting the player focus on bosses while the dark energy kills smaller enemies.

Accessories

 * Damage increasing accessories are a must for events. Among the most effective are the, and , , ,  and.
 * Wings such as, or the.
 * is very useful for healing between waves.
 * Variants of the may be useful to prevent knockback.

It's recommended to reforge all accessories to Lucky, Menacing, or Warding.