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In 1666, an angel blessed the rats whose flight had given him enough warning to save his books from the Great Fire of London.
The rats were delighted by their new longer lives, better health, and immunity to plague, and were sure to spread themselves out as far as they could go. They took to ships, crossing channels and seas, until they could be found anywhere a rat could easily get. They took with them a certain memory of the one who had blessed them, and certain habits that came along with the blessing - for example, they tended to steer clear of chewing books.
In 1793, a demon blessed the rats (descendents of the earlier, angel-blessed ones) who led him to the angel's cell in time to save him from the French Revolution.
From then on, the rats became unstoppable. Smarter, faster, and wilier than ever, they could get into any place, track down any creature or person, find any item that they wanted, learn any information available. Wards that were set to keep out unholy creatures bounced harmlessly off the angel's blessing. Wards that were set to keep out holy creatures bounced harmlessly off the demon's blessing. Miracles set merely to keep out 'normal' creatures bounced off both.
The palest of the rats learned to creep in the angel's footsteps when he went to report, and skitter all over the too-cold-too-empty-no-food-boring-place he spent time in.
The darkest of the rats found that they could scamper openly past the demon's heels once he was through the entrance to his warm-dark-loud-smells-interesting reporting place and none of the crowds there took the slightest notice, even when one brave rat made her nest in some of the older reports and raised several broods there.
The rats took all of this skill, and all of the knowledge they gained and formed themselves into an army for hire - a group of rat mercenaries as it were.
They called themselves 'The Shenanigans.'
And by 1910, when the demon woke up from his very long nap, the Shenanigans were perfectly placed to tell him everything he wanted to know about the angel.