Summoning the Old One's Army in Terraria represents the pinnacle of endgame preparation, a ritual that transforms the final frontier of space into a battleground for cosmic horror. This formidable event provides the essential materials required to craft the armor and weapons necessary to defeat the Lunatic Cultist and subsequently face the Moon Lord. Successfully initiating this army requires meticulous planning, precise execution, and a thorough understanding of the game's complex spawning mechanics, turning a simple world into a stage for an epic celestial conflict.
Preparation and World Requirements
Before even considering the call to arms, your world must meet specific criteria to ensure the event can proceed without critical failure. The primary requirement is defeating at least one mechanical boss, either The Destroyer, Skeletron Prime, or Queen Bee, which unlocks the necessary NPCs and items. Your world must also be set to Hardmode, as the event is locked behind this difficulty threshold. Furthermore, it is crucial to ensure you are not in a Small or Medium world, as the Old One's Army is exclusive to Large and Expert/Professional worlds, providing the expansive battlefield required for the extraterrestrial invasion.
Acquiring the Slime Crown
The physical key to initiating the event is the Slime Crown, a quest item obtained through a specific chain of actions involving the Dye Trader NPC. The process begins by purchasing a Slime Mask from the Dye Trader, who appears randomly during the day. Equipping this mask and then submerging yourself completely in water during a Blood Moon will transform you into a Blue Slime, allowing you to enter the Underground or Cavern layer. Here, you must locate and defeat the Queen Slime, which has a small chance to drop the Slime Crown upon its death, making the encounter a mandatory stepping stone.
Locating the Dye Trader
Ensure the game is in Hardmode.
Wait for the Dye Trader to spawn, which is rare and requires a 1 in 15 chance per day.
Purchase the Slime Mask from his inventory.
Executing the Transformation
Equip the Slime Mask in your accessory slot.
Find a body of water deep underground, away from surface interference.
Submerge entirely to transform into a Blue Slime.
The Ritual of Summoning
With the Slime Crown in your inventory, the event can be triggered at any time outside of a boss fight, provided the other conditions are met. The most reliable method is to use the Crown at the very edge of the map, specifically at the far right side of the screen within the horizontal spawn zone. This location is optimal because it prevents the Monolith from spawning inside blocks, which would instantly despawn the event. Using the item summons a Monolith, a massive alien structure that immediately alerts the Cultists and initiates the three-wave assault.
Understanding the Three Waves
The Old One's Army is divided into three distinct waves, each introducing new and more dangerous enemies that test your defensive capabilities. The first wave focuses on basic flyers and melee attackers, serving as a warm-up to gauge your setup. The second wave escalates the threat with stronger units and the introduction of the Ancient Cultist, a dangerous spellcaster. The third and final wave is the most chaotic, unleashing the powerful Devourer of Gods and the ancient wyrm, Aquatic Scourge, culminating in a intense survival scenario where every damage-dealing accessory and weapon is crucial.
