Corruptor's Scourgestone

A Corruptor's Scourgestone is a small talisman taken from the most powerful Scourge. The Argent Dawn will accept Scourgestones as proof of your willingness to fight against the Scourge. Scourgestones may be turned in to Nicholas Zverenhoff at Light's Hope Chapel, Argent Officer Garush at The Bulwark, or Argent Officer Pureheart at Chillwind Camp.

Source
This item drops from Scourge bosses in the Western Plaguelands, the Eastern Plaguelands, Scholomance and Stratholme. You must have the Argent Dawn Commission, Seal of the Dawn, or Rune of the Dawn equipped to pick up the Scourgestones.

As a quest objective

 * Corruptor's Scourgestones