670 AD. The body of an unknown young noble is found murdered not far from Cashel. The only clue to his identity is the emblem he was wearing – the emblem of the ruling house of the neighbouring kingdom of Laigen. When King Colgú of Cashel sends his sister, Fidelma, and her companion, Eadulf, to investigate, they are propelled into one of their most dangerous and perplexing mysteries. They find themselves struggling with a tangled skein of murder and intrigue; a bewildering conspiracy and, moreover, physical jeopardy. Is the eruption of violence that has broken out in the west of the kingdom connected? Who is the fanatical figure who leads the violence, claiming to have been summoned by "the seventh angel" to drive the "impure of faith" from the land? What is the mystery that connects the dead noble, a murdered alcoholic priest, and a menacing abbot who has built his abbey into a military fortress rather than a religious community? What evil stirs out of the shadowy Gleann na nGeilt, the Glen of Lunatics? Fidelma herself becomes the victim of abduction and it is up to Eadulf to find and save her from imminent death. Should he beware of the attractive Princess of the Éile or her neighbours, the Osraige, a border people with a reputation for being untrustworthy? Who is the handsome young poet who seems to be a key to bringing together the strands of a complex plot to overthrow the Eóghanacht ruling house of Muman of which Fidelma’s brother is the head? This is one of Fidelma’s most baffling investigations yet.