Delirium was introduced in Patch 3.10 (March 2020) and remains an important cornerstone of PoE's endgame nowadays. This League Mechanic revolves around the Mirror of Delirium; when used, this triggers a zone-wide effect called Delirium Fog. The Fog empowers existing enemies and spawns new ones while granting extra rewards. Related drops include Delirium Orbs for Maps, Simulacrum Splinters to enter the Simulacrum and Cluster Jewels to extend the Passive Skill Tree.

On entering a Map with the Delirium League Mechanic, you see a big Delirium Mirror right at the entrance of the Map. Walk through the Mirror to activate the mechanic. This triggers a Fog that starts spreading throughout the entire Map, buffing existing and spawning new monsters.

The map will be covered in Fog and a new icon appears in the bottom left of the window. Those are your reward icons, which include several items from League Mechanics or specific item groups such as Currency or Map Fragments. The reward icon fills up as you kill enemies in the Fog stacking up until there are no monsters left in the Fogor the Fog disappears.

After hitting 3 levels in a reward type, another reward icon pops up next to the first one starting with level 1. Icons progressively fill as you kill monsters in the Fog, so every 3 levels another type of reward pops up next to the latest one.

The Fog also spawns eggs on the ground in addition to the extra monsters, you can also find eggs on the ground. Stepping on the eggs results in even more monsters. There are two types: white and red. The former spawn Delirium monsters for more loot; the latter should be avoided as they chase you down but drop nothing.

The Fog dissipates progressively from the Mirror at the beginning of the map towards the end of the Map, so your goal is to kill as many monsters as you can while inside. If you step out of the Fog or it outpaces you, a timer appears at the top of the screen. You have 5 seconds to get back into it before the Delirium ends completely.

To help you navigate through the Map without having to backtrack, certain League Mechanics either slow down or completely stop the Fog's movement for their duration.

There is a distinction between Delirium created by a Mirror versus an Orb. Maps crafted with Layers of Delirium have a fixed Delirium effect rather than scaling with the distance. On the other hand, the Delirium Fog doesn't dissipate, so take as much time as you like to finish these Maps. In those cases, the special rewards are dropped whenever you fill the bar.

All Delirium Orbs function the same way, but there are different reward types indicated by an icon. Similar to crafting your Maps with Cartographer's Chisel or Orb of Alchemy, you can apply a Delirium Orb on top of the quality, quantity and rarity. The Orb turns a Map into a Delirium Map and shows the type of reward you get from the Delirium mechanic. Up to 5 Orbs can be applied to a single Map, each adding 20% Delirium Effect. The higher this effect, the harder the Map becomes.

You also have a higher chance to see Delirium Bosses by using more Orbs (there are 2 different Bosses). They drop additional Simulacrum Splinter or occasionally specific unique Cluster Jewels.

Large Cluster Jewel, Medium Cluster Jewel and Small Cluster Jewel are a class of Jewels in Path of Exile that drop from Delirium encounters. Those jewels are socketed in the outermost sockets only of the Passive Tree to extend it by basic Nodes, Notables, and even Keystones mod pool.

You can craft Cluster Jewels in the same way as normal Jewels. All Cluster Jewels can have a maximum of four Affixes - two Prefixes and two Suffixes. The mod granted by the small passives determine the explicit Mod Pool of the Jewel, as well as add effects to the small passives in the Cluster.

There are also preferred sizes when it comes to the number of Passive Skills. In most cases, you prefer 8 Passive Skills for Large Cluster Jewels to spare Skill Points and some rarer Builds prefer up to 12 points because they stack the "Waypoints" instead of Notables!

In this case, most Builds prefer 4-5 Passive Skills because as you can see in the picture above, you can spare exactly 1 Passive Skill point when you travel over Notables instead Waypoints!

A unique Small Cluster Jewel can only be obtained by killing Omniphobia, Fear Manifest and Kosis, The Revelation in Maps affected by Delirium or in the Simulacrum.

Simulacrum Splinter drops from Delirium encounters. A full stack of 300 completes a Simulacrum. Using the Simulacrum Map Fragment will opens 6 portals to an alternate version of one of the following Act 6-10 Campaign towns: Lunacy's Watch, The Bridge Enraptured, The Syndrome Encampment, Hysteriagate or Oriath Delusion. You encounter the Strangely Familiar Entity before every wave of the Simulacrum. You also notice the monster modifier and Delirium reward type, which increase with every wave. 

There are a total of 30 Waves and each wave adds a 5% Delirious effect, up to 100% at Wave 20. Because of how Delirious scales monster damage reduction, there is a massive difficulty spike on Waves 19 and 20. From that point on the effect doesn't scale further but mods still become slightly harder every time:

If you want to run Simulacrums for profit, you need to understand some points beforehand:

Delirium offers two Unique bosses in Maps and Simulacrum. The chance to do so scales with the Delirious effect. The bosses also drop Unique Small Cluster Jewels in Maps and Medium or Large Unique Cluster Jewels exclusively in Simulacrum.

The following unique items were added in Patch 3.10 as part of the mechanic. Since then, these items have been added to the core drop pool but are still thematically connected to Delirium: