The Traitor's Blade - Kevin Sands

Christopher’s homecoming takes a sinister turn when a murderous conspiracy is uncovered in this fifth novel of the award-winning Blackthorn Key series.Christopher, Tom, and Sally are back in London at last. Everyone is relieved at their return—the plague has ended, and the king, pleased with their service, offers a surprising reward. But trouble has followed them home... First, an old friend is ambushed and left for dead. Then an anonymous letter arrives at Blackthorn—a mysterious warning hidden inside a riddle and secret code. As Christopher and his friends begin to investigate, they soon discover they’ve stumbled upon a plot to kill the king...and anyone else who dares stand in the traitors’ way.