Before the fall of Arlathan, even before Arlathan itself, the civilization of the elves stretched across all of Thedas like a great, indolent cat. This armor was made for temple guards in a time when the Creators still spoke to the elves. The techniques of its forging, even the name of the metal it is forged from, have long since faded from memory. —From Codex entry: Ancient Elven Armor
In the Ruined Temple, after lighting the Brazier, you will unlock a room with several traps, two ballista, and four archers. Directly after this room is a room with a large round dais on the floor. On the far right side of this room is the chest that contains this item.