Whatever the delve of Molavar was in the past, the flaming pith of its mountain hermitage has made it now unfit and unsafe for mortal habitation. The clever Thalmor however found another use for the lost settlement, turning it into a prison for a necromantic vampire convicted of a wicked tally of heinous crimes. It’s jailers are the flaming atronachs, imps and colossi summoned to these broiling halls by the indicters, and perhaps even the undead denizens of the long lost settlement; although they may well have been risen by the necromancer herself.
But was it really so clever of the Thalmor to imprison an immortal monster as a punishment rather then destroy it? It is certainly a decision I believe that no human court would have decreed. Perhaps for the long lived Altmer the idea of an eternity of imprisonment seems a far worse fate then quietus itself. But surely for such a punishment to be logical then the convicted would have to be capable of feeling guilt, remorse, and contrition.
Instead this monster under the mountain is only capable of feeling resentment, rage and enmity toward the living. It is almost inevitable that the undead creature will eventually escape, or be released by intent or misadventure to ignite blistering vengeance upon Tamriel. So it falls to me to correct the Thalmor’s folly and turn this monster to ash.