This is the helmet of Garahel, the elven Grey Warden hero who killed the archdemon Andoral during the infamous battle of Ayesleigh in 5:20 Exalted, the battle which ended the Fourth Blight. While Garahel's other weapons and armour are enshrined at Weisshaupt Fortress, Garahel's helm fell off during the hero's final battle with the archdemon and was subsequently lost. 
It was enough to level what buildings were not already destroyed by the endless battle we had fought, enough to knock horses and ogres aside as if they were little more than parchment. Even at my distance, the force struck me like the blast of some great storm. The darkspawn around us felt it too, as savage desperation turned to sheer terror.
A great pillar of energy rushed up into the dark clouds, the blackness that had gathered with the horde and blocked out all glimmer of hope. When we stood again, we saw the first rays of sunshine peeking through those clouds and we let out such a cheer of joy and relief that it shook the very earth. I joined the others as we searched for Garahel, but as the eve approached all I found was his enchanted helm. It was not until much later that I heard his body had been retrieved, flung to the far side of the battlefield by the Archdemon's death throes. My friend, this elf who helped us unite the lands and cleanse Thedas of the darkspawn scourge will always be remembered. I swear it.—Excerpt from a letter written by the Grey Warden Prosper, 5:24 Exalted
—From Codex entry: Garahel's Helm