The Dying Light

From PlotBunnies

Everybody knows vampires are hurt by sunlight. What they don't know is that in the late 18th century, a vampire known as Doctor Carlyle created a solution. (He was, after all, a genius by virtue of centuries of experience) He managed to create a retrovirus which could cause vampires' skin to absorb the holiness in sunlight and re-emit it as visible light. So instead of turning to dust, when the sun shines upon them a vampire's skin will glow faintly. The clearer the sun, the brighter the glow.

However, this method wasn't without its drawbacks. You see, if the light from a glowing vampire falls upon another, they will glow in response too. And they get brighter and brighter, until they move away. So they can't spend too long in each other's company, unless in darkness. And mirrors, well ... a vampire illuminated by his own reflection will get brighter and brighter, until eventually the intensity is just too great and they burst into flame. If the reaction is started by holy light (sunlight) falling upon them, they can explode in seconds, especially if the mirror is close.

So one team came up with a radical new idea - they would break up a church's stained glass windows, already sanctified, and each piece acts as a tiny mirror or lens. Fired from a demi-canon, these fragments scythe through a vampire army. First they glitter, as the fragments of holy pane settle on them. And then they burn in hell, destroyed by the light of heaven.