I had a surface fort inhabited by more than a hundred happy dwarves. When one of them was found drained of blood, I quickly set to work identifying and sequestering away the vampire.
Six years later, the fortress found itself under siege by goblins riding a phalanx of war cave dragons. The fighting was brief, but the invaders never realized that deep below the crypt was a single undead dwarf toiling away with pick, axe, and anvil, living in quiet seclusion and exceptional thirstiness. (Being a vampire, his thirst for booze was replaced with one for blood, but he still suffered from alcohol deprivation...) That dwarf might have heard the distant slaughter, but he ignored it as part of a surface world he was doomed never to see.
However, months afterwards, as vampire dwarf toiled away, he, for the first time, felt as though he was being watched. Looking over the cave lake, he saw the distant skittering form of the giant ant creature that lived down there with him, but as always, the dumb beast ignored him. He returned to his labors, figuring that it must have simply been the insect's presence that disturbed him.
The next day- insomuch as days had any meaning in the perfectly dark underworld in which the vampire lived- was silent as usual, the tomblike fortress he had built himself echoing only with his faint footsteps. But some time later he glimpsed what could have been a mist pouring from a dark doorway, self-luminous in the utter darkness that only undead eyes could pierce. As he turned the corner, he saw only more dark corridors.
One does not live a thousand years and travel ten thousand miles without learning of the nature of the world. The vampire now knew that surely something was amiss. He took a break from his tasks and commenced a fine-toothed search of his apartments and tunnels. He looked under stones, sealed small gaps, and even poured out the worthless booze from his small stockpiles.
However, he found nothing. After weeks of search, the vampire picked up his pick once more and continued working.
As he dug into the rock, he struck a vein of soft, flaky stone. Some slate or other fine sediment that came away in great sheets. Then, his pick sunk into the stone particularly easily, and stuck. A rumble sounded, the first sound of such volume heard here since the early days of the earth. The stone wall trembled, and then fell away. Within the newly opened caverns were the silvery shadows of a fortress, arrayed for battle. With ghostly cries, sounding like the whistling wind far above in the open, bloodstained entrance to the crypts, the dead fortress moved to battle.
Centuries later, a lost traveler in the desert hills might stumble into a ruined fortress full of bleached bones. This fortress might be filled with plentiful ammunition and fine crossbows sufficient to arm an army, but only dust would occupy the adobe abodes. The communal well might still function, its deep, deep cistern of cold water having sustained other travelers before, and the earthen ramparts might even have housed the occasional bandit group or outlaw gang.
But if that lost traveler were to find the lever that opened the first gateway, then were he clever enough to pick the lock on the fine doorway behind that, and then if he should wander through a labyrinth of pitch-black stone, he would find something very odd...
An unused bedroom.
An office with parchments turned to dust by centuries.
A forge and workshop where once a strange mind labored.
And a seam of soft stone where the head of a copper pick lies buried in rubble, and a dry, dry, dry corpse lies petrified in a state of total fear, its skull almost normal except for the feral teeth of a carnivore.
TL,DR: Vampire scared to death by ghosts. Was not expecting that.