Down Memory Lane
Many mechanics in Path of Exile, most notably Zana's Map Device in the endgame, allow you to access the mechanics of past Challenge Leagues. This is a brief list of those leagues and their mechanics, starting with the most recent leagues and going backward. Some Leagues in the distant past were limited to Softcore or Hardcore players only, but this is no longer the case (and there are no restrictions on Softcore or Hardcore players obtaining their respective items).
Note that most League mechanics can still be accessed in some fashion, even if the League itself was not integrated into the core game. The exceptions are Bestiary and Legacy leagues, whose mechanics are no longer accessible.
Legion League (June 2019 - September 2019)
They believed themselves the pinnacle of civilisation, but that height toppled their empire.
Mechanic: Players encountered Timeless Monoliths in each zone. Interacting with a monolith would reveal a battle between two armies frozen in time, and players had a short time to damage enemies to 'unfreeze' them. After the time limit ran out, players fought whatever enemies they unfroze.
Fate: Legions were added to the core game with Patch 3.8. They now appear randomly in Maps and through other means.
Synthesis League (March 2019 - June 2019)
Even shattered as I am, I know that memories floating around like this is not... normal. I believe something terrible has happened to me.
Mechanic: Players met Cavas, the Forgotten Spirit, in each zone. Cavas opened portals to memories that, if successfully cleared within a time limit, were added to a special inventory. Players could then use the captured memories as "tiles" to cross a void to reach rewards.
Fate: Synthesis was almost entirely removed from the game after its end. A few of its bosses are now accessible through Zana missions.
Unique Items: Synthesis unique items still drop from its bosses, as accessed through Zana missions. Fractured and Synthesized items, which were unique to Synthesis league, are no longer obtainable.
Betrayal League (December 2018 - March 2019)
Some of the members we have come across... well... they have come back from the dead. I don't have any better way of putting it... This must be why they call themselves the Immortal Syndicate.
Mechanic: Players met Jun Ortoi, an NPC leading a fight against the Immortal Syndicate, composed of previous friendly NPCs who are now members of a hostile Syndicate. Players could manipulate the hierarchies and relationships among the Syndicate in order to control the rewards earned from it.
Fate: Betrayal mechanics were added to the core game with Patch 3.6.
Unique Items: Betrayal Unique items are obtainable from the Syndicate just as they were during the League.
Delve League (September - December 2018)
The darkness within those walls is unnatural. It does not fade in the presence of light... It retreats. Like a living beast. I'd not wander too far from the lamps if I were you.
Mechanic: Players met Niko the Mad, an NPC intent on delving into the mysteries of a the deep Azurite Mine. The Mine is an infinite procedurally-generated dungeon with various treasures and monsters, which got better and harder as you decended deeper into the zone. Delves were powered by Voltaxic Sulphite, a resource found in the open world.
Fate: Delve was added to the core game with Patch 3.5.
Unique Items: Delve Unique items are obtainable from the Azurite Mine just as they were during the League.
Incursion League (June - August 2018)
No wonder it's lost! It's in the middle of the jungle!
Mechanic: Each zone contained an NPC named Alva Valai. Alva opened Incursion portals to the past construction of a Temple, and the structure and rewards of the Temple could be modified by what you killed in Incursions. After 11 Incursions, you gained access to the Temple proper, and could loot whatever you had unlocked in the past.
Fate: Incursion was removed briefly, then returned to the core game in Patch 3.5.
Unique Items: Incursion Unique items are obtainable from the Temple just as they were in Incursion League.
Bestiary League (March - May 2018)
This one is captured! Einhar will take it!
Mechanic: Every "Beast" (basically, animal-like creatures, not humanoids, golems, etc) in the game became capturable using special net items. Capturing a Beast sent it to your Menagerie, a storage area for captive Beasts, and the captive Beasts could be used in special item recipes to add mods to an item, create special new items, etc.
Fate: The Bestiary league was poorly received. For a while it was removed from the game, even as a Zana mod, but it was eventually returned to the core game in Patch 3.5 in a greatly simplified form.
Unique Items: Bestiary items are obtainable from rare Beasts just as they were in Bestiary league.
Abyss League (December 2017 - February 2018)
For aeons, in the deepest, blackest reaches, they have festered and multiplied.
Mechanic: Each zone contained an Abyss. Once activated, an Abyss would crack the ground, moving along the floor and spilling out numerous enemies. Killing these enemies would keep the Abyss open, and keeping it open long enough would yield a chest (common) or an opening to a special Abyssal Depths zone (rare).
Fate: Abysses were added to the core game, and continue to drop associated Unique items.
Unique Items: Kept, but limited to Abysses (and cannot come from an Orb of Chance). Some items associated with Abysses could drop from regular Abyss monsters during the League, but became limited to the Abyssal Depths zone when Abysses were added to the core game.
Harbinger League (August - November 2017)
Mechanic: Each zone contained a Harbinger, which would summon waves of enemies. Defeating those enemies damaged the otherwise immune Harbinger, which would drop special currency items on death. These new currencies added mechanics previously unavailable, enabling new crafting options. Using one of these currencies, a Harbinger Orb, could upgrade a map into the special Beachhead map, which was filled with Harbingers and dropped pieces of special Unique items.
Fate: Harbingers were not added to the core game, but some of their associated currency items were.
Unique Items: The actual Uniques themselves remain limited to The Beachhead, itself only accessible by using a Harbinger Orb on a Map. The Orb of Annulment was added to the core game; the other special Harbinger currency items (including the Harbinger Orb necessary to access the Beachhead) remain Harbinger-specific drops (generally accessible only via a Map Device mod).
* No, that's not the site glitching - Harbinger items' flavor text used special symbols.
Legacy League (March - July 2017)
Legacy pays homage to the leagues and items of the past.
Mechanic: Special items called Leaguestones could drop from enemies. Three of these stones could be equipped in a special panel, and would modify any zone you entered while they were equipped (consuming some of the Leaguestone in the process). Each Leaguestone was associated with one of the previous leagues, allowing you to open zones associated with any of those Leagues and their mechanics. In addition, a rare drop called an Ancient Reliquary Key enabled access to "Relic" versions of Unique items with stats those items had had in the past (before being changed in one or more patches).
Fate: No Legacy content was added to the core game (even as a potential mod for the Map Device). This wasn't due to poor reception, so much as that this league was a loot bonanza intended as a "Last Hurrah" for the game as it was prior to the major Fall of Oriath update.
Unique Items: Legacy had no Unique items of its own aside from the Leaguestones and Reliquary Keys, neither of which was added to the core game.
Breach League (December 2016 - February 2017)
We are within her reach, and when the time is right, she will reach into us.
Mechanic: Each zone contained a Breach. Breaches caused an expanding ring to spread across the map, revealing numerous enemies associated with one of the five Breachlords (and thus with one of the five Damage Types). Killing these enemies could drop Splinters, which could be combined to open a portal to one of the Breachlords' home domains in order to fight them.
Fate: Breaches were added to the core game, appearing randomly in some Maps.
Unique Items: Kept, but limited to Breaches (and cannot come from an Orb of Chance).
Essence League (September - November 2016)
Encounter groups of monsters trapped by an ancient force, break them out to steal their Essence then use these to craft powerful items.
Mechanic: Each zone contained an Essence monster trapped in crystal. Releasing the monster allowed you to fight it, and it would drop one or more of a variety of Essence items. Essence mechanics are quite complex, and are covered on a separate page of this guide.
Fate: Essences were added to the core game as-is.
Unique Items: Essence had no league-specific Unique items, and Essences remain in-game.
Prophecy League (June - August 2016)
I sense you have questions. For a gift of silver, I will answer them. But you seek more than answers.
Mechanic: A new NPC named Navali was introduced. For the cost of Silver Coins (dropped somewhat commonly by enemies), she would grant Prophecies with a wide variety of mechanics. Prophecy mechanics are complex, and are covered on a separate page of this guide.
Fate: Prophecies were added to the core game as-is, although Silver Coins became rarer.
Unique Items: Prophecy-specific Uniques remained in-game, although most of those Uniques were (and remained) specific to Prophecy-spawned monsters.
Perandus League (March - May 2016)
Now you have the look of a connoisseur, an educated appreciator of all things exquisite and dangerous.
Mechanic: Perandus Chests filled with a special currency called Perandus Coins could be found throughout Wraeclast. These coins could be traded to a randomly-appearing NPC named Cadiro Perandus, who would offer items in trade (with a different offer on each encounter).
Fate: Perandus mechanics were not added to the core game.
Unique Items: Perandus-specific Uniques were not added to the main game, and can drop only in areas affected by Zana's Perandus mod.
Talisman League (December 2015 - March 2016)
We have tried to paint our future in words. Now we shall paint with inks of savagery.
Mechanic: Monsters could be found carrying Talismans, special amulets that always dropped corrupted (but which came with very powerful Implicit modifiers). Talismans came in tiers, and could be upgraded by sacrificing five talismans of the same tier at special Stone Circles found throughout the world. Sacrificing five Tier 3 Talismans would open a portal to a special boss with unique drops.
Fate: Talisman mechanics were not added to the core game.
Unique Items: Talismans, and Talisman-specific uniques, were not added to the core game, although some later leagues (like Legacy's leaguestones or Incursion's Temple of Atzoatl) could offer access to Talismans.
Warbands League (July - October 2015)
To fight an enemy on their lands is a tactical mistake.
Mechanic: Special Warbands could be found throughout Wraeclast, belonging to one of four clans (associated with Fire, Cold, Lightning, or Chaos damage). Warbands could drop magic items with special modifiers and special Uniques specific to each Warband.
Fate: Warbands were added to the core game in a limited fashion, as the bosses of certain Maps.
Unique Items: Warbands Unique items remain limited to their associated Warbands, but are still attainable by running the right Maps.
(Warbands was a Softcore-only League, and ran in tandem with Tempest League)
Tempest League (July - October 2015)
An angry god is one thing,but you should really fear a god with an unpredictable sense of humour.
Mechanic: Each zone was affected by a Tempest. Tempests created circles on the ground which would, after a few seconds, apply one of a variety of effects (some beneficial, some harmful) to both players and monsters in the area.
Fate: Tempests were not added to the core game as a random effect, but can be triggered by various means (most notably certain Prophecies). A Tempest being present, however, does not mean the zone is flagged as Tempest-associated (that is currently only an effect of the associated Map Device modifier).
Unique Items: Tempest Unique items can only drop in Tempest-associated zones, currently only available as an effect of Zana's Map Device.
(Tempest was a Hardcore-only League, and ran in tandem with Warbands League)
Torment League (December 2014 - March 2015)
A fearsome foe's inner torment spills out into the world.
Mechanic: Tormented Spirits appeared randomly in various zones. Spirits would flee from the player for a period of time, seeking out Rare or Unique monsters to possess (and granting a buff to lesser monsters they passed through). Buffed monsters had increased drops and gained special abilities. If not killed and unable to find a Rare or Unique monster to possess, Tormented Spirits would vanish.
Fate: Tormented Spirits were added to the core game.
Unique Items: Kept, but can only drop in zones flagged as Torment-associated (which does not include all zones with Tormented Spirits). Like most other League-specific Uniques, the zone must be explicitly tagged by Zana's Map Device's Torment mod.
(Torment was a Softcore-only League, and ran in tandem with Bloodlines League)
Bloodlines League (December 2014 - March 2015)
A bond of blood breaks.
Mechanic: Magic (blue-rarity) monsters came with special Bloodlines modifiers, highlighted in purple on the list of modifiers visible by mousing over the monster.
Fate: Bloodline modifiers were added to the core game, although not every magic-rarity monster has a Bloodline. Some maps come with a mod that causes all Magic monsters to have one.
Unique Items: Kept, but can only drop in zones flagged as Bloodlines-associated (which does not include all zones with Bloodline Magic monsters). Like most other League-specific Uniques, the zone must be explicitly tagged by Zana's Map Device's Bloodlines mod.
(Bloodlines was a Hardcore-only League, and ran in tandem with Torment League)
Rampage League (August - November 2014)
What is greed but trading a mountain of death for a sliver of life?
Mechanic: Killing monsters started a Rampage count on the player, which rose with each kill, similar to other 'kill streak' mechanics in other games. At certain counts, a player would trigger some new effect or gain some new buff that would persist for the duration of the Rampage.
Fate: Rampage was not added as a default mechanic, but is now given by certain special items (including those specific to the Rampage league).
Unique Items: Kept, but can only drop in zones flagged as Rampage-associated (which does not include simply playing with a Rampage item). Like most other League-specific Uniques, the zone must be explicitly tagged by Zana's Map Device's Rampage mod.
(Rampage was a Softcore-only League, and ran in tandem with Beyond League)
Beyond League (August - November 2014)
Soon, the pleas for mercy were muted. Only one voice remained. Laughing.
Mechanic: Killing monsters could leave behind small blood orbs. Enough blood orbs in close proximity (about a screen) would summon a weak pack of Beyond monsters, which would in turn leave larger grasping mouths on the ground in their wake. Concentrating enough of those would summon even stronger monsters, eventually including special Beyond bosses.
Fate: Beyond was not added as a default mechanic, although Maps can trigger Beyond's mechanics via a mod ("Slaying enemies close together can attract monsters from Beyond").
Unique Items: Kept, but can only drop in zones flagged as Beyond-associated (which does not include all maps that summon Beyond demons). Like most other League-specific Uniques, the zone must be explicitly tagged by Zana's Map Device's Torment mod.
(Beyond was a Hardcore-only League, and ran in tandem with Rampage League)
Ambush League (March - July 2014)
A trap is set, a lure is placed, but stronger foes lie in wait.
Mechanic: Players could find Strongboxes in various zones around Wraeclast. Different Strongboxes contained different kinds of items, but all of them would spawn packs of monsters around the Strongbox when interacted with, and could not be opened until those monsters were defeated.
Fate: Strongboxes were added to the core game.
Unique Items: Kept, but can only drop in zones flagged as Ambush-associated (which does not include all maps that have strongboxes in them). Like most other League-specific Uniques, the zone must be explicitly tagged by Zana's Map Device's Ambush mod. (One of the associated items, the Drillneck quiver, was added to the core game, but the other two remain restricted.)
(Ambush was a Softcore-only League, and ran in tandem with Invasion League)
Invasion League (March - July 2014)
A terror from the past returns to cause harm in a place it never called home.
Mechanic: Both normal monsters and bosses could be found in zones other than their normal homes.
Fate: Neither mechanic was added to the core game. Invasion Bosses can occur as the result of various effects (most notably Prophecies and a Map Device modifier); unusual monsters for a zone appear routinely as a common affix on a Map.
Unique Items: Kept, but can only drop in zones flagged as Invasion-associated (which does not include all maps that have Invasion bosses or unusual monsters in them). Like most other League-specific Uniques, the zone must be explicitly tagged by Zana's Map Device's Invasion mod. (One of the associated items, the Rearguard quiver, was added to the core game, but the rest remain restricted.)
(Invasion was a Hardcore-only League, and ran in tandem with Ambush League)
Domination League (October 2013 - February 2014)
Open the eye of the storm.
Mechanic: Domination introduced Shrines, which provide a powerful buff to monsters near them. Clicking the Shrine would transfer that powerful buff to you for a short time.
Fate: Shrines were added to the core game.
Unique Items: Kept, but can only drop in zones flagged as Domination-associated (which does not include all maps that have Shrines in them). Like most other League-specific Uniques, the zone must be explicitly tagged by Zana's Map Device's Domination mod.
(Domination was a Softcore-only League, and ran in tandem with Nemesis League)
Nemesis League (October 2013 - February 2014)
You draw the ire of another, imbued with golden power.
Mechanic: Rare (yellow-rarity) monsters came with an extra Nemesis modifier, which appeared in a bright orange on the list of modifiers visible by mousing over the monster.
Fate: Nemesis modifiers were added to the core game, although not every rare monster has one. A common affix on Maps causes all rare monsters in the map to have a Nemesis modifier.
Unique Items: Kept, but can only drop in zones flagged as Nemesis-associated (which does not include maps that simply have the affix that causes Rare monsters to have Nemesis mods). Like most other League-specific Uniques, the zone must be explicitly tagged by Zana's Map Device's Nemesis mod.
(Nemesis was a Hardcore-only League, and ran in tandem with Domination League)
Anarchy League (June 2013 - October 2013)
One like you roams the land, but they are just the first of many.
Mechanic: The Anarchy League introduced Rogue Exiles, enemies that use player skills in a build an actual player might make. Rogue Exiles can be found scattered in various zones.
Fate: Rogue Exiles were added to the core game.
Unique Items: Kept, but can only drop in zones flagged as Anarchy-associated (which does not include maps that simply contain a Rogue Exile or have affixes that force them to spawn). Like most other League-specific Uniques, the zone must be explicitly tagged by Zana's Map Device's Anarchy mod.
(Anarchy was a Softcore-only League, and ran in tandem with Onslaught League)
Onslaught League (June 2013 - October 2013)
The attack, cast and movement speed of monsters of Wraeclast has been increased.
Mechanic: The Onslaught League buffed all monsters with a 20% increase to Movement, Attack, and Cast Speed (i.e., the Onslaught buff).
Fate: Monsters do not have the Onslaught buff by default. However, Onslaught has been added as a keyword used by numerous effects, typically granted to the player, with the same bonuses as above.
Unique Items: Kept, but can only drop in zones flagged as Onslaught-associated. Like most other League-specific Uniques, the zone must be explicitly tagged by Zana's Map Device's Onslaught mod.
(Onslaught was a Hardcore-only League, and ran in tandem with Anarchy League)