I'm horrible at remembering items because I make my own, but I have an Ebony Helm of Destruction that I found somewhere. I doubt it's unique - I think it was just randomny generated. I'm sure you know that 2 magic items respawn at the Shrine of Mehrunes Dagon every few ingame weeks. I also found a Shrouded Cowl that gives me 20% bow damage - again: no idea from where I got it.
The Ebony Helm is a leveled random item, and the Shrine is a bit more of a challenge than I would care for at mid-teen level, particularly for random items. You got the Ancient Shrouded Cowl at Hag's End, which bugs me because I was already there for the Predator's Grace boots, but never found a cowl. It's leveled all the way up to 35%.
You should be careful with that Shrouded Cowl. It's glitched in a *lot* of ways. It can allow you to use dragon priest masks, Falmer helmets and circlets all at once, combining all effects, but then can randomly stop working completely, forcing you to wear a second item with it just to get its own effect to work. The face mask portion of the cowl doesn't work for Dunmer, Argonians, Khajjit or Imperials, and also erases their hair. It's also been known to permanently wipe weapon and armor effects to zero, forcing you to back up to a save prior to putting it on.