The "Hunt for the Harbinger" is a story questline (Patch 10.2.7) that sets the stage for the War Within expansion.

: Completing the questline grants the "Hunt for the Harbinger" achievement, a unique transmog, and a void-themed mount skin for your Winding Slitherdrake. 🧊 Minecraft: L'Enders Cataclysm Mod Guide :: How to earn the Harbinger title - Steam Community

: Complete a master-level Hunt without any team deaths.

: They change tactics and gain new abilities as their health drops. Destiny 2: The Harbinger Mission & Title

: It features heavy platforming (the "dam" section) and several "survive" arenas where you must defeat waves of Taken enemies. The "Harbinger" Title

The "Harbinger" refers to both a challenging exotic mission and a prestigious player title. The Harbinger Mission (Hawkmoon)