I lucked out and just happened to kill a random Goblin Ambusher who dropped the key. While wandering around the north side of Wadi I found a chest, used the key, and received the shield. I'm leveling a thief, so I just decided to hold it until I could use it.
I was reading about the Dangruf Wadi Treasure Key and a poster said they got one of these by using said key on a treasure chest they found somewhere on the north side of Wadi. See the post here.