The Celebration
The Monsters sat down to their celebratory feast. They toasted themselves with goblets of blood, then began eating the suckling long pork in earnest. As the screams filled the air, they congratulated each other on their success. They'd done it; their evil stupidity had at last destroyed civilisation and obliterated the human race.
Outside the barred cave people got on with their lives, secure in the knowledge that the Monsters were finally gone. The Monsters had told so many lies for so long that they believed whatever they said. But they were the only ones who did: their success was entirely in their own deranged minds, and since they thought it was true they no longer had to try and implement their idiotic plans of annihilation.
The only thing the Monsters had conquered was themselves.