Old One's Army



The Old One's Army is a unique Dungeon Defenders 2 crossover event that can be triggered by placing an Eternia Crystal on the Eternia Crystal Stand. Waves of enemies begin spawning from portals on both sides of the crystal, seeking to destroy it. The objective is to protect it until all waves are cleared – if the crystal is destroyed, the event ends.

There are three different versions of the event: It first becomes available after defeating the Eater of Worlds/Brain of Cthulhu (first tier), receives a substantial increase in difficulty after defeating any mechanical boss (second tier), and upgrades to its final version once Golem has been defeated (third tier).

Clearing later waves after wave one rewards the player with Defender Medals, a unique currency that can be used to purchase special weapons and armor from the Tavernkeep NPC. This gear is specifically suited for the event, due to its exceptional level of difficulty. Not only are its enemies extraordinarily resistant, it also employs numerous anti-cheat/exploit mechanisms, such as preventing the player from placing or removing blocks or liquids for its duration.

Summoning
To initiate the Old One's Army event, an Eternia Crystal Stand must be placed first. An Eternia Crystal must then be placed on it by pressing the key on the stand. Both the crystal and its stand are purchased from the Tavernkeep NPC for coins. The Tavernkeep can be found randomly in a world after the Eater of Worlds or the Brain of Cthulhu has been defeated.

Terrain requirements
Summoning this event has some unique requirements for the terrain around the summon area. If they are not fulfilled, the Eternia Crystal cannot be placed successfully, and a status message appears informing the player that the surface is inadequate.


 * A minimum clearing of 61 tiles horizontally is required 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.
 * A minimum clearing of 10 tiles vertically is required (i.e., between the "floor" and the "ceiling", if there is one).
 * Additionally, tiles above the floor may not be immediately next to the Eternia Crystal Stand.
 * The floor may not have holes wider than 1 block.

The player can build up to a height of three tiles before blocks are considered obstructing the necessary space. Platforms and inactive blocks do not count as part of a flat floor, and will count against the three-tile height allowance; but floor blocks with Actuators on them that are solid when the crystal is placed will count as flat, and can be successfully actuated after the event has started. In addition, closed trap doors are considered flat, and can be successfully opened and/or reclosed after the event has started.

Waves
The event consists of a set number of waves, with each wave becoming progressively more difficult (see wave chart). At the end of each wave, there is a 30-second countdown to the next wave, signified by a timer appearing over the Eternia Crystal.

The event comes in three possible tiers, depending on game progression when the event is triggered:


 * Tier 1 consists of five waves, with one or more Dark Mages serving as mini-bosses in the final wave. Tier 1 is triggered when the event is summoned before any mechanical bosses have been defeated. The final wave will only end when a Dark Mage is killed; however, note that a single kill does not give enough points to instantly end it unless the wave is mostly already finished. Completing Tier 1 awards 5 Defender Medals in total.


 * Tier 2 consists of seven waves, with one or more Ogres serving as mini-bosses in the final wave. Dark Mages will not appear in this tier. Tier 2 is triggered when the event is summoned in a Hardmode world after at least one mechanical boss has been defeated. The final wave only ends upon the death of an Ogre. Completing Tier 2 awards 25 Defender Medals in total.


 * Tier 3 consists of seven waves, with the boss Betsy spawning in the final wave. The Dark Mage and Ogre will also spawn during the previous three waves. Tier 3 is triggered when the event is summoned after Golem has been defeated. The final wave ends when Betsy is defeated. Completing Tier 3 awards 100 Defender Medals in total. In Expert Mode worlds, an additional bonus of 30-49 Defender Medals is dropped by Betsy's Treasure Bag. Furthermore, in Master Mode worlds, the boss relics will only drop during this wave.

In addition to the difference in bosses, the common event enemies also differ between Tiers, with stronger statistics and a more menacing appearance in higher tiers.

Defender Medals and Sentries
The completion of each wave, beginning with the third, awards the player Defender Medals, a special currency used to purchase items from the Tavernkeep NPC that can help players defeat future Old One's Armies. Among them are special sentry summon weapons, which are fueled by Etherian Mana, a resource that drops only during this event. Etherian Mana allows multiple sentry minions to remain placed simultaneously and permanently until the event is over, with more Etherian Mana allowing more sentries to be placed. All Etherian Mana disappears once the event is over, whether uncollected or within the player's inventory.

At the beginning of the event, the player will receive 10 Etherian Mana, allowing them to place one sentry. Note that sentries not obtained from the Tavernkeep (such as the Queen Spider Staff) do not and cannot use Etherian Mana, but can be summoned separately.

Each sentry type available from the Tavernkeep (Flameburst, Explosive Trap, Lightning Aura, Ballista) comes in three different tiers: Rod, Cane, and Staff, with the Staff being the most expensive and the most powerful.

Once the event is completed successfully in a world, the weapons can then be used outside the event with normal summoning rules.

Drops
Each boss and mini-boss in the event has a trophy (10% chance) and a mask (14.29% chance). Betsy also drops a Treasure Bag in Expert Mode (100% chance).

Several enemies also drop pets, vanity items, and powerful weapons and accessories. All enemy banners dropped during this event are non-functional, and do not provide the player with any of the usual defense or damage boosts that other enemy banners provide.

Enemies will not drop biome-specific drops, such as Biome Keys, or coins, even when wearing the Lucky Coin or its derivatives. They also do not drop Presents during Christmas, nor Goodie Bags during Halloween.

Anti-cheat mechanisms

 * The Creative Shock debuff prevents placing or mining blocks during the event. It also prevents using Buckets to spill or collect any liquids.
 * All blocks: Blocks cannot be used to completely prevent event enemies from advancing. Old One's Army enemies are able to go to enter a "ghostly" state where they can move through blocks. They initially spawn in this state, then re-enter it whenever there is no clear path between them and the player. For example, if the player enters a tunnel below the arena, enemies will fall through the blocks to enter the tunnel.
 * : Event enemies are not affected by Teleporters, rendering them useless for the event. However, players are still able to use them.
 * : Event enemies take no damage from lava, rendering it useless for the event.
 * : Honey Blocks will not slow down the enemies, and only hinder the player's collection of Etherian Mana crystals.
 * : On the, Cannons and Bunny Cannons deal no damage to event enemies, rendering them useless for the event. However, Explosive Bunnies will still deal damage to enemies if placed.
 * : While traps will damage event enemies normally, they are less efficient than sentries; Traps can additionally make sentries less effective, since trap attacks cause invincibility frames in the enemies they hit, while sentries do not. Traps may still be useful in areas out of the reach of sentries.

Things that are still possible:
 * can be used to transform the arena after the event has started. Keep in mind that walls can result in the enemies simply moving through, but this can help protect the crystal from projectiles. (these might not work)
 * See the Actuator drop strategy on the page Guide:Old One's Army strategies for a specific implementation that circumvents the mechanisms.
 * can place temporary ice blocks.
 * can be used to move Dirt Blocks in the area.
 * Weapons and tools that create placed blocks are usable and will produce blocks. However, these blocks cannot be removed with tools until the event is over.
 * will still produce placed Sand Blocks when fired.
 * The and  can be used, and will produce placed Dirt Blocks.
 * and Explosives can destroy blocks that are not explosion-proof.
 * can be used to move Etherian Mana towards the player. Note that they do not move enemies, only the player, as usual.
 * : As of 1.4.0.1, Etherian Mana can be placed in a chest, Piggy Bank, Void Vault or similar tiles left unopened between events, allowing it to be reused for future events.
 * It is possible to open a Piggy Bank, Void Vault, or chest with Autopause enabled to stop it from disappearing in between events.
 * Minecart Tracks are allowed in the arena, allowing the player to use Minecarts to run over enemies.

Equipment chart
The Old One's Army event brings associated equipment. The armor and sentry weapons are available from the Tavernkeep in exchange for Defender Medals, while other weapons and accessories are dropped by event enemies. Though all of the equipment is coordinated with other equipment of the same class, the only specific benefit from this coordination is the buffs granted to each sentry by the corresponding armor set bonus; neither the weapons nor the accessories are involved in any additional unique bonuses, and so can be used with any other armor or exchanged for any other weapon at the player's preference.

Tier 2 items become available after one Mechanical Boss is defeated, and Tier 3 items become available after Golem is defeated.

Tips

 * Because of the Creative Shock debuff, any building work to prepare the arena must be completed before. It is not possible to place or remove blocks or furniture while the event is happening, not even between waves.
 * During the event, players can combine two different kinds of sentry with different limits:
 * The sentries (Queen Spider Staff, the Staff of the Frost Hydra, the Rainbow Crystal Staff, and the Lunar Portal Staff) work as normal: The player pays a modest amount of mana, and each last the usual 2 minutes.  By default the player can only summon one at once, but Tavernkeep armor and/or Ogre drops will let them summon more.
 * During the event, the Tavernkeep sentries are not (and cannot be) summoned with regular mana. Instead they cost 10 points of the event's Etherian Mana.  The Etherian Mana supply is the only limit to how many the player can summon, and they will last until the event ends.  (Outside the event they work like any other sentry.)
 * In general, any player will struggle to finish an OOA invasion without a Tavernkeep sentry for the tier; for later tiers having a set of Tavernkeep armor will also help. The player gets a Tier 1 sentry for free;  Saving up  will let them buy a Tier 2 sentry before facing the Tier 2 army. If they can save up a full, they will also be able to buy Tier 2 armor up front! During Tier 2, they should focus on collecting at least for a Tier 3 sentry -- will let them get the armor as well. Note that both the sentries and the armor are also quite useful for regular gameplay.
 * As the presence of water does not impact terrain requirements, an arena filled partially or completely with water can allow use of certain mounts or items to allow increased 3-dimensional mobility. Superheated Blood can be obtained as soon as lava fishing is available and should be combined with a Gills Potion for a submerged arena. Shrimpy Truffle also provides increased damage under these circumstances and can be obtained, with difficulty, during the Tier 2 timeframe. A submerged arena should be well-lit with Coral Torches and carefully placed; when located in, for example, the Jungle or Ocean biomes, Arapaima or Sharks can spawn and attack the Eternia Crystal.
 * Boulder Statues on a Timer can be remarkably helpful, if the player can keep out of the boulders' way. Note that boulders can also damage the Crystal!
 * If the player is in a Hardmode world but has yet to defeat a mechanical boss, they can easily farm the event with a Blessed Apple or Ancient Horn mount. Simply run back and forth. The damage the mounts do while charging will one-hit kill all enemies besides the Dark Mage. (This does not work if the world is on expert mode)
 * Players can use a one-block thick ceiling to their advantage, however, as it allows air enemies to be targeted by dedicated elevated sentries, where they are more likely to score hits.
 * Platform blocks can support sentry summons. By using free-standing platforms, the player can elevate their sentries without blocking off their own attacks. Flameburst Sentries placed 20-25 tiles up from the crystal will much more easily dispatch Wyverns before they attack.
 * If the player uses the Clinger Staff, it can act as a barrier. Combined with a single Lightning Aura sentry, it can take down one side of the army until around Wave 4.
 * The Nimbus Rod is extremely effective in Tier 1, as it kills all enemies in one hit except the Dark Mage. If the player places the cloud towards the crystal-facing side of a pillar, no enemies will get through aside from the Dark Mage, which the player can handle. The player will need to replace the cloud every so often, though.
 * In pre-hardmode, a Water Bolt or Demon Scythe can be extraordinarily useful due to their piercing abilities and high damage.
 * The Seedler, when used with a completely flat arena, can be useful because the projectiles from the weapon can shoot out horizontally, allowing the player to stand in the center and have the projectiles fire towards both sides, killing enemies from both portals.
 * Similarly, the Space Gun (best when combined with Meteor Armor) can be fired from behind one portal and across the entire arena, hitting enemies as they spawn from both portals. This is an extremely effective, near-effortless way to deal with Tier 1 waves.
 * Once you've cleared Tier 2 a few times and (hopefully) gotten a Tome of Infinite Wisdom, it's right-click tornado does much the same thing as the Space Gun did for Tier 1. Combine with a mana star magnet effect to make up for the high mana cost, and Tier 2 becomes an absolute breeze.
 * If the player enters a tunnel below the arena, enemies will fall through the blocks to enter the tunnel; this behavior can be exploited to make them unable to damage the Crystal as they move themselves behind barriers to attack the player instead.
 * The player should avoid the Ogres' spit attack as they will slow the player down significantly.
 * In Tier 2, killing the first Ogre early on may cause another one to spawn before the wave ends.
 * Hoiks can be used to some extent to fling Etherian Mana the player's way, but only to the right. A slower alternative are Conveyor Belts. This is of limited use since the break timer between waves offers plenty of time to gather Etherian Mana manually.
 * Hoiks can be used on the Eternia Crystal itself, and a hoik-loop can move the Crystal continuously so that it cannot be easily hit.
 * Due to the sheer amount of enemies, piercing ammunition, such as Jester's Arrows, can be quite useful. The Flying Knife is also very useful.
 * By placing a hoik facing away from the crystal and replacing it with a platform, enemies will not recognize this as an impassable wall, and will be permanently stuck being pushed back by the hoik with no way of reaching the crystal. This, of course, only works for grounded enemies.
 * Although the second tier is unlocked after only one mech boss has been defeated, it will be hard to fight until after all of the three bosses, as even adamantite or titanium armor and gear may not be enough to progress to the later waves.
 * However, using the above strategies and/or the actuator drop strategy, It is possible and easy to defeat the second tier with weaker armor and weapons.

Trivia

 * The Eternia Crystal slowly develops cracks as it gets more damaged.
 * In the Bestiary, some of the enemies will appear as their Tier 3 variant, even before reaching the tier.
 * In the files an unused enemy known as DD2 Attacker test which looks identical to the Angry Tumbler enemy can be found.