"The Wretched Abyss" as seen in game.
He is encountered in Septimus Signus's Outpost after giving Septimus the Dwemer Lexicon. He wishes the Dragonborn to replace Septimus Signus as his emissary after Septimus has opened the lock on the Dwarven/Dwemer lock-box. If Dragonborn is installed, he will appear as a greenish-black cloud with a clump of tentacles and floating eyes, much like he appears during the Dragonborn main questline. If talked to after completing Dragonborn's main questline, the Wretched Abyss comments on the defeat of Miraak.