(from ebookjapan):

At the end of the Tokugawa shogunate, the Imperialists and the Sabaku (feudal shogunate) were fighting each other with their own espionage networks.

At such a time, there was an agent who belonged to neither faction and was commonly known as a "Karasu". The shogunate, cornered by Satcho, planned to assassinate Kaishu Katsu, who insisted on a bloodless surrender of the castle for the sake of the peasants.

What did "Karasu" do, exactly?