THIS BOOK MAKES NO GODDAMN SENSE.
I see a lot of reviewers waxing lyrical about the portrayal of the corruption of Horus, but here's a (BARELY) condensed timeline. I'm not going to mark any spoilers, the book's five years old and the Heresy itself was laid out in broad strokes twenty years ago.
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Last year: 6319. Horus is a Good Guy. A really good guy. Loken is attacked by a demon. Horus explains about demons, says that they're real and malignant.
A couple months ago: Erebus, a Bad Guy who doesn't even come across on the page as a competent liar, shows up. Horus is still a Good Guy. Erebus steals something. They have to fight their way out of hostile country.
Monday: "Horus Rising" ends. They're going to Davin.
Tuesday:
"Horus, I'm sorry, I was corrupted by Chaos! Watch out for Chaos!" (Dies)
"What's Chaos?" (Falls over)
Wednesday:
"Get me 50ccs of Phlebotinum, stat! He's not stabilizing! If only I knew about the weapon that wounded him, can you go get it for me?" (Horus bleeds)
"Sure, but don't do anything crazy while I'm gone." (Loken leaves)
"I know magic and superstition are forbidden nonsense, but maybe we could take him to these wizards I know?" (They take Horus to wizards and seal him in a room, bleeding to death, with no medical care, because wizards)
Thursday:
"Hey, this is one CRAZY dream, hey dead friend, didn't expect to see you here!"
"Hi, I am TOTALLY your dead friend, now I am going to tell you a pack of obvious, easily caught out lies, starting with 'I am your dead friend!'"
"No, Horus, I am your brother primarch, don't listen to him, it's not really him and you know it!"
"Well, that's true, I'm not really your dead friend and I admit I've fed you a pack of lies, but you should totally believe me, because he's using magic, and Magic Is Bad, except when I use it to appear in your dreams."
"You make a solid point, self-admitted liar! I should turn evil now!" (Horus turns evil)
Meanwhile, in a library...
"This ancient text mentions Horus! It must be prophecy, not Aleister Crowley verbally masturbating all over the page!" (A demon appears, then is slain by an obvious and visible miracle)
"Pay no attention to what you all just saw! Magic is still not real!"
Friday:
"Hey legion, you know that guy not all of you trust? He lied to me in a magic dream, but I'm sure he has our best interests at heart, let's turn evil!" (Evilness happens)
"Let the obvious murders commence!" (Murdering commences)
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Okay, more seriously - he's a 300-year-old demigod who is on record as a master of strategy and an excellent reader of men, but when he repeatedly catches not-really-his-dead-friend in a series of blatant lies, and his brother primarch shows up to warn him about the lies, he makes such an obviously bad decision that any chance of this man being chosen as humanity's greatest warlord, never mind overthrowing a 30,000-year-old physical god who could literally unmake him with a thought, becomes too great a suspension of disbelief to bear. We are supposed to believe Horus can see through every stratagem, except when Erebus makes a really obvious series of blunders and even dream-Horus calls him on them over and over again.
There was so much potential there, especially given what we know about, say, the change in Henry VIII's personality after his jousting injury. He could have been healed, but the healing gone wrong in so many more subtle and insidious ways, his doubts over his own mortality stoked and a fear of his eventual death leading him to a pact with the Ruinous Powers. Instead, we get the obviously least trustworthy character in the history of literature up to and including Loki themselves, overseeing supposedly near-instantaneous corruption of a 300-year-old demigod whose idea of subtlety is to walk into a room and say "HEY, YOU ARE A GOOD WRITER, PITY NO ONE WILL EVER BE ABLE TO READ IT!" and to orchestrate a conspiracy where one of his own troopers shoots a covert target dead center of mass, square-on, with his own bolter. If he's going to be that obvious, why not just arrange a Stalin heart attack and shoot him in a basement?
This book reads like they tried to shortcut the Heresy and, like Fred Savage in "The Princess Bride," 'get to the good parts.' I can understand that from marketing to gamers, but after the very slow burn of "Horus Rising," this book feels rushed and makes no goddamn sense.