Conqueror's Thassarian's Battlegear

Conqueror's Thassarian's Battlegear is the lower rank of the Thassarian's Battlegear set, the Alliance death knight Tier 9 damage dealing set. The middle set is Triumphant Thassarian's Battlegear, the upper set is Triumphant Thassarian's Battlegear (heroic).

The Horde equivalent to this set is Conqueror's Koltira's Battlegear.

Source
Thassarian's Battlegear of Conquest pieces can be purchased from Dubin Clay in Dalaran for various amounts of Emblem of Triumph.