The One Thousand: Book Two: Team of Seven - The One Thousand, #2

It is the late 1960s... In this sequel to "The One Thousand" a team composed of humans and benevolent aliens hunt for murderous, alien-possessed convicts with enhanced powers who have escaped from prison.  They discover that this fellowship of psychopaths is preparing an elaborate party for hippies and other street people in a remote mansion built to simulate a Medieval castle, and that they are planning to slaughter everyone who attends.  Now the seven are faced with the task of locating the mansion and stopping the killers...