Pocket Fel Spreader

You need a way to detect demons before approaching Doomsayers. Demon Hunters can use Spectral Sight (a toggleable ability that can be kept active indefinitely). Any class can craft or buy Elixir of Detect Demon from an Alchemist using Gromsblood x2 and Crystal Vial x1 — it is also available on the Auction House, though it only lasts 10 seconds with a 30-second cooldown. Leather, Mail, or Plate wearers can farm Cursed Vision of Sargeras from Illidan Stormrage in Black Temple at a roughly 20% drop rate, which provides an indefinite passive demon detection.

There are 14 Doomsayers spread across Stormwind and Orgrimmar. In Stormwind, find them at 69.07, 32.96, 52.11 75.29, 71.73 45.41, 45.67 51.27, 40.33 63.78, 44.33 69.11, 52.03 84.92, 52.52 63.90, 65.53 47.63, 57.75 53.02, 33.9 67.92, 40.7 46.9, 39.76 77.72, 48.92 57.10, 44 64, 51.59 55.97, and 46.37, 61.69. In Orgrimmar, check at 68.1, 17.1, 78.6 37.3, 68.6 42.7, 72.7 47.8, 67.6 58, 77 63.6, 66.1 71.3, 63.6 77.7, 61.3 75, 55.6 68.5, 51.2 70.6, 50.2 86.7, 44 81.2, 47.4 49.6, 53.9 52.9, and 59, 53. Note that one Doomsayer is located in the Cleft of Shadow at 52.87, 48.46, which is not visible on the main map.

With your demon detection active, speak to each Doomsayer and look for the special dialogue option: "There's something not quite right about you.." — selecting this will immediately transform the Doomsayer into a Dread Infiltrator. If none of the Doomsayers show this option, each one refreshes their dialogue approximately every 15 minutes, so cycle through them. If a Dread Infiltrator is reset, it despawns back into a Doomsayer and will not have the option available again right away.

The Dread Infiltrator has approximately 16 million HP and uses abilities similar to a Dreadlord, including a Slumbering Fog that floats around — stepping into it will put you to sleep and deal 7,000 damage per tick for 4 ticks, along with a vampiric ability. Defeating it has a chance to drop the Pocket Fel Spreader, though the drop is not guaranteed.