In your travels you may come across mysterious entities with names in a language you don't recognise - Harbingers from an unknown realm. These Harbingers can create and command monsters from the area, and empower them with their own abilities. You can't directly harm the Harbingers yourself, but defeating a Harbinger's monster allies hurts the Harbinger as well. Successfully defeating a Harbinger earns you currency shards of both existing currency types and currency types exclusively available from these entities.
One of these currency types may lead you to the source of the Harbinger incursion. If you can halt their arrival, you'll find pieces of shattered unique items that can be arranged to form powerful Harbinger-summoning uniques.