The Purifier

The Purifier is a gun with a bonus to critical strike rating. It is acquired via Argent Dawn quests turn-ins.

Source
The quests are given by Quartermaster Miranda Breechlock. She has a Quest for each tier of faction (Friendly, Honored, Revered and Exalted). It requires a turn-in of a large number of Insignia of the Dawn and Crusade. The Quests are:
 * Epic Armaments of Battle - Friendly - 110 Insignia of the Dawn and 110 Insignia of the Crusade
 * Epic Armaments of Battle - Honored - 75 Insignia of the Dawn and 75 Insignia of the Crusade
 * Epic Armaments of Battle - Revered - 45 Insignia of the Dawn and 45 Insignia of the Crusade
 * Epic Armaments of Battle - Exalted - 27 Insignia of the Dawn and 27 Insignia of the Crusade