Vigilante

Before the Warrant quests become visible, you must first accept the quest A Key Threat. The Warrant quests then appear on a poster next to that same NPC at 48.7, 57.7 in Tazavesh, the Veiled Market.

Each week, one of six Warrant quests is available at 48.7, 57.7 in Tazavesh, the Veiled Market: Warrant: Arcana-Monger So'zer, Warrant: Grubber, Warrant: Shatterpulse, Warrant: Xy'vox the Twisted, Warrant: Purple Peat, or Warrant: Hollowbane. You can complete only one per week, so the full achievement requires a minimum of 6 weekly resets.

Each Warrant quest leads through 3 additional quests and ends with a key reward used to summon and kill a rare enemy. The keys — Purple Peat Cell Key, Xy'vox Refuge Dampener, Grubber Lure, Arcane Lure, Shatterpulse Cell Key, and Hollowbane Portal Key — are not bind on pickup and can be purchased from the Auction House, and buying them still grants progress toward the achievement.

After completing all 6 Warrants, the Terror of the Night mount is delivered via in-game mail from the Postmaster rather than granted directly, so check your mailbox even if your inventory is not full.