** "The Darkest Hour" is book six of the (4-author as one) Erin Hunter tribal cat Warriors series, linked by a theme of proving loyalty. This story narrative is fairly straightforward, but has several surprise murders. Prologue warns us about enemy Shadow clan chief Tigerstar's plot. Dreams and prophecies from Star clan warn Thunder clan families leader Firestar. The villain takes over River clan and wants the whole forest in his new unified Tiger clan. He invites tyrant dictator Scourge to bring Blood clan, twoleg territory unrelated hungry strays, who arm their claws with dog teeth, as enforcers.