Harbinger league

"Mysterious Harbingers have appeared throughout Wraeclast. Harbingers use a language never before heard by the people of Wraeclast. Their true nature remains a mystery to be solved."

- Description on the official Path of Exile website

The Harbinger league and Hardcore Harbinger league were a pair of challenge leagues. The leagues started on 2017-08-04 along with the release of Version 3.0.0 and ended on 2017-12-04.

This league introduced Harbingers, otherworldly enemies which can only be defeated by destroying its minions. Harbingers drop shards of currency items, including shards for currency not normally found in Wraeclast. All characters and stashes were moved to Standard league and Hardcore league respectively when the leagues ended. Characters that died in the Hardcore Harbinger league were immediately moved to the permanent Standard league.

Challenges and Rewards
The Harbinger leagues included 40 optional challenges. Completing them awarded the following microtransactions:

Modifiers
All areas have:

Items
These items are "exclusive" to Harbinger monster of the Harbinger leagues. After the league end there are additional ways to encounter Harbinger monster or spawning loot container that contains Harbinger league reward directly, such as Incursion, Delve, Legion and Blight.

Currency shards
Shards are primarily dropped by Harbinger. Some shards have additional ways to obtain, such as from vendor recipe.

Currency
These items are primarily acquired from combining currency shards:

In addition,, and  were exclusive to Harbinger league in version 3.0. However, since 3.1, 3.7 and 3.14 respectively, they are added to the main loot table.

Unique items
These items were exclusive to the Harbinger leagues. Except the unique map, they still exclusively from combining the unique pieces. The unique pieces themselves are dropping from The Beachhead maps. While the creation of that map is "exclusively" from the Harbinger currency:.

Unique maps
Beachhead map are primarily acquired from using on maps:

Unique pieces
Unique pieces are acquired from Beachhead maps only: