Jossed, The Heart of Darkness is the real Big Bad and was impersonating the Ancestor during the first phases of the fight, who was really dead.

He, or at least a good imitation of him, is the Mouth of Sauron for the Heart of Darkness, though he's a Punch-Clock Villain only bound to his role by the Heart's power. The Dragon theory is correct, in a sense.It's far more likely that the Dark God that the Ancestor summoned, and the barbed arch symbol represents, is the Big Bad, but the Ancestor might prove to be The Dragon.The Ancestor may very well be still alive, waiting in the deepest pit of the Darkest Dungeon, and serving as the Final Boss. Making a relative of him inheriting the Darkest Estate could have been a twisted plan to lure some sacrifice fodder right into the hands of the cultists, as well as getting rid of potential Spanners In The Works (the Prophet, the Hag, the bandits.) while expecting the Darkest Estate's hazards and dwellers to be tough enough to prevent the heroes from gaining control of the domain. There isn't any proof that the letter written during the opening was sincere. The Ancestor is the Big BadThe Ancestor was involved in very shady business with dark magic and necromancy.

Darkest dungeon tv tropes