Gallowglass by Barbara Vine (Pseudonym of Ruth Rendell)

When Sandor snatched little Joe from the path of a London Tube train, he was quick to make clear the terms of the rescue. 'I saved your life,' he told the homeless youngster, 'so your life belongs to me now'. Sandor began to tell him a fairy-tale: an ageing prince, a kidnapped princess chained by one ankle, a missed rendezvous. But what did this mysterious story have to do with Sandor's preparations? Joe had only understood his own role: he was a gallowglass, the servant of a Chief...