Wretched Abyss is an apparition of Hermaeus Mora. He appears as a purple portal with a black center, similar to that of summoning creatures from Conjuration. His appearance changes with the Dragonborn add-on installed to a being of grotesque eyes and tentacles.
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 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.