Everything has a cost in blood. Often that cost is farmed out – the blood of the third world pays for our cheap electronics, and their hazardous waste, for our cheap clothes, our cheap cars, or cheap… well, everything. But not all costs can be outsourced. And you did not really imagine you could pile so many people into such a small space without paying a cost? Without appeasing some dark gods for the favour of resting your city on their shoulders?
The undying ones live in the darkness below. They were there before we came, slumbering. Now they lie, restless in the dark; hungry. Their dark magic keeps the city from collapsing in on itself, from yielding to the impossibility of its existence. It greases the cogs and gears of this living engine, teeming with its chaotic human fuel.
And they are paid in blood.
A deal was struck long ago; small sacrifices, for the greater good. A secret, steady supply of fresh blood, to keep the undying heart of the city beating. And so every few months, late at night, a railway car will take a turn down an uncharted tunnel. It stops at a station unmarked on the map; an elegant, gothic station, more temple than transit-hub. The train will wait until its passengers grow restless, and disembark, looking for a way out, and its doors shut behind them. They will make their way up the sole central staircase, and there find themselves in the hall of the undying ones; a vast, impossible space beneath the city; a cathedralic space. Their screams echo up the tunnel, but never carry far enough for anyone to hear.
The keepers of this compact, generations of secret-keepers, have no choice but to ride the rails as well; the only way their consciences will allow them sanity. You can spot them, if you look, when your train comes to a sudden halt in the middle of a tunnel, or should the lights flicker; the terror wild in their eyes. A weak smile on their lips when the train lurches back to life. Should you ask, they will simply mumble something about claustrophobia.
And they are not lying, for the mouths of the undying are confined spaces indeed.