To defeat the Darkness in the castle (after you have blown open the gate), go up to every chair you see in the main room. Each chair will prompt you to activate it. Note: This does give away your Darkness powers for the time being. After the Darkness appears as a red/evil Jackie, put any guns away so that just your hands are out. This will stop his attacks. Then, walk up to him without any guns out, and a prompt should appear to "Accept Him". The fight will then be over.