Every vampire in the Bloodbound Houses belongs to a family, and every family is a House. The Houses are old — some of them close to a thousand years — and the world they hold together is governed less by law than by a very long memory.
Six who remained
The living Houses are Marchand, Voss, St. Croix, Tahir, Calderón, and Krios. Each keeps a different corner of the world and a different temperament: Marchand in Paris, all red velvet and old money; Voss in the Carpathians, on a mountain it has not left in a very long time; St. Croix in New Orleans, the only House that kept a seat in the New World. They do not love one another. They have simply agreed, for seven hundred years, not to be the one who breaks the peace.
The seventh
There were seven. One was burned out of existence on a winter night in 1326, and the world agreed afterward not to speak of why. That burning is where the trilogy begins — because the House did not end as cleanly as everyone was told.
Why it matters to the romance
This is a romance, and the Houses are the reason it is a hard one. To love across a House line is to owe two loyalties that do not agree. The people in these books fall for each other while standing on opposite sides of something that has not been settled in seven centuries. That is the engine of the whole thing: not whether they want each other, but what wanting each other costs.
Nobody in these books is only a villain. Everyone is somebody's family.