It began with a low vibration, subtle, rhythmic, almost hypnotic.
At first, the citizens of London assumed it was another midnight train rumbling beneath the old city.
A distant storm offshore. A trick of wind.
But the sound grew deeper. Heavier. More industrial.
Like the heartbeat of some leviathan god stirring in the heavens.
Then the sirens began to wail.
They were supposed to be relics, antiquated, ceremonial, last tested during drills.
Few Londoners had ever heard them in earnest.
Not like this. Not shrieking in unison from every corner of the city like the howling of banshees.
Above, the night sky was black.
No stars. No moon.
But to those who looked up, the darkness was moving.
Dozens, no, hundreds, thousands of sleek shapes slid across the void like knife-blades.
