Successfully complete the indicated task to unlock the corresponding legendary 
armor:

  Blood Dragon: Use the DLC to get the first piece, and the rest will be in 
  Bodhan's shop in your camp.
  Juggernaut: Examine the three tombstones in the Brecilian Forrest to get the 
  first three pieces. Successfully complete the "Jug" quest in the Ruins to get 
  the final piece.
  Legion Set: Pieces are spread throughout the Deeproads.
  Reapers Vestment (best robe): Purchased it at the Wonders Of Thedas.
  Robes Of Possession: Successfully complete Morrigan's character quest by 
  killing Flemmeth. They can then be found in the chest in Flemeth's hut.
  Wade's: Wade of Wade's Emporium in Denerim will make you three sets of armor, 
  two of which will use 3x Drake Scales, and the other will need the Dragon 
  Scale. The first two sets are leather armor, and the third is your choice of 
  medium, heavy, or massive. You can get lots of Drake Scales and the Dragon 
  Scale during the "Urn Of Sacred Ashes" quest. Note: Make sure you always pay 
  the extra gold when asked to get the best results.