Doff is an Asura located in the Vloxen Excavations. Mamp sent him there to guard the gate leading to Vlox's Falls. He'll spawn out of nowhere and open the gate for you when you encounter him for the first time.
"Mamp, the Asura honcho, set me here to guard this gate leading back to the Tarnished Coast. He gave me this Phantasmal Cloak to conceal myself, but the smoke is almost unbearable. How does it work, you ask? Badly, if at all. And the vapors make my eyes water. Of course you are welcome to use this gate. It rarely kills anyone using it..."