So let's steal an idea from the original Gothic story "Castle of Otranto" by Walpole and Shunned House of Lovecraft - an undead physical being buried under the house - and still growing underground, until it assumes grotesque and monstrous proportions. It "sleeps" but its psychical presence is warping and destroying anyone living in the house or its vicinity.
http://www.dagonbytes.com/thelibrary/lo ... dhouse.htm
"We never - even in our wildest Hallowe'en moods - visited this cellar by night, but in some of our daytime visits could detect the phosphorescence, especially when the day was dark and wet. There was also a subtler thing we often thought we detected - a very strange thing which was, however, merely suggestive at most. I refer to a sort of cloudy whitish pattern on the dirt floor - a vague, shifting deposit of mould or nitre which we sometimes thought we could trace amidst the sparse fungous growths near the huge fireplace of the basement kitchen. Once in a while it struck us that this patch bore an uncanny resemblance to a doubled-up human figure, though generally no such kinship existed, and often there was no whitish deposit whatever. .On a certain rainy afternoon when this illusion seemed phenomenally strong, and when, in addition, I had fancied I glimpsed a kind of thin, yellowish, shimmering exhalation rising from the nitrous pattern toward the yawning fireplace, I spoke to my uncle about the matter. He smiled at this odd conceit, but it seemed that his smile was tinged with reminiscence. Later I heard that a similar notion entered into some of the wild ancient tales of the common folk - a notion likewise alluding to ghoulish, wolfish shapes taken by smoke from the great chimney, and queer contours assumed by certain of the sinuous tree-roots that thrust their way into the cellar through the loose foundation-stones."
So, what is the source of that presence or curse? There was once a cruel belief or custom: when laying a foundation of any great building, a living sacrifice was buried under it, so that its spirit would guard it, and its life-force strengthen the builiding. Of course, when building a palace or an especially important house, the sacrifice had to be human. Sacrifices of infants were deemed especially valuable, as their life-force is especially fresh and plentiful.
http://www.paolaraffetta.com.ar/extensi ... ifice.html
http://books.google.pl/books?id=h7mlAHi ... &ct=result
http://theshadowlands.net/places/Netherlands.htm
"Denekamp - Singraven Manor - At the beginning of the sixteenth century Singraven Manor became shortly the residence of some Franciscan nuns. As a building sacrifice a nun was bricked up alive. Since then there was not much happiness at the house anymore. Owners and family died young, got in serious financial trouble or were the victime of some deadly accidents. One of the owners burned alive after an oil lamp fell over and set the unhappy man on fire. The ghost of the nun still haunts the house, which is now a museum. Her appearance predicts disaster and shame. "
http://mek.niif.hu/02700/02790/html/139.html
"Once more they took counsel, all twelve master masons,
How to stop walls crumbling, how the building hasten;
Till at last agreed they, came to this solution,
All between themselves they made a resolution:
“Any of our wives who be the first arriver,
Gently we should take her, throw her in the fire,
Mix with lime her ashes, tender ashes softly
For to strengthen with it Déva’s castle lofty.”"
Now let us imagine a noble necromancer, who wants to ensure the future domination of his house (ie, family) over the whole Mordent. When he begins to build his house, he secretly, using his magic, abducts his chief enemy and his three children and ritually kills him as a building sacrfice, burying their bodies beneath the cornerstone. His spell was to ensure that his House (ie family) should rule over all great houses of Mordent. The dead were transformed into a single mindless force, a spirit-guardian, bound to ensure the fulfillment of the spell, but at the same time hating their creator, his family, and all inhabitants of the building. Moreover, the curse was planned to grow with time, since all its victims were to be imprisoned in the House, serving to strengthen it.
When later the last descendant of the necromancer was killed for treason, the spell lacked the original focus, and transformed into ensuring that the actual physical House should rule Mordent. Its malice was now free to seek revenge on the inhabitants, and as it grew in power, it began to gain control over all great houses of Mordent, destroying the noble families living there.
And during all that time, the bodies of the original sacrfices have dissolved together into one putrid mass, which gradually formed into one, monstrous body - the physical embodiment of the Curse. And as the Curse gained power, the body grew, until it could barely fit, doubled together, under the whole House.