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.