The Dead Caste

The Dead Caste is a side quest encountered during A Paragon of Her Kind.

Walkthrough
Four pieces of equipment must be acquired to complete this quest -- all of which are located within The Dead Trenches area of the Deep Roads. Four of the five pieces are different components of the Legion of the Dead Armor, and one is the Dead Caste Insignia.
 * The Boots of the Legion are in a sarcophagus in the room immediately to the left of the tunnel exit from the first bridge area where the legionnaires are fighting the darkspawn.
 * The Gloves of the Legion are in a sarcophagus in the room containing a fire-breathing dwarven statue trap directly opposite from the boots.
 * The Armor of the Legion is in a sarcophagus in the room with the skeletons
 * The Helm of the Legion is in the Legionnaire Altar within the Legionnaire Shrine.

After all four pieces have been acquired, the codex will reveal a location found inside the tunnel leading up to the Broodmother. Obtain the Dead Caste Insignia from the coffin and return to the Orzammar Shaperate to interact with the "Memories" book on the right-hand side of the shaper.

The armor does not need to be equipped.

The Gangue Shade
Donning the entire set of armor is required to initiate the Gangue Shade unmarked quest. Activating the Legion of the Dead Relic (found in the room where you acquired the helmet in the central area) while the entire harness of armor is equipped by any party member summons the Gangue Shade, which appears to be a fairly garden-variety shade with a unique name. When defeated it drops no items* (unless using the workaround for Dead Metal Bucket). The encounter does not trigger a quest or journal entry, although Codex Entry: The Gangue Shade does appear. *It should be noted that, on the 360, it is possible to get Chainmail (Silverite) as a drop from the Shade. Also verified on PC, possible to get Chainmail (Dragonbone) - (some players got silverite on PC).