I hated it. It's not as bad as any of Matt Ward's stuff, but still.
Most of the fic reminds me of a bad comic book where everything is based around the next big fight scene - doesn't matter if it makes no sense - doesn't matter if it's wildly out of character for everyone involved - the "awesome" fight scene is going ahead whether or not it makes any sense whatsoever.
For me, this story was wallbanger after wallbanger. I think the worst was when it turned out that Kage of the Last Chancers was the father of the reborn Emperor. I was like, "No! Kage is an unrepentant psychopathic mass murderer with latent psychic powers and a cacodaemon infesting his brain. Also, he's dead. He committed suicide in order to A) repent for his actions while he was being mind-controlled by the cacodaemon and B) bring down Herman von Strab. Yeah, Kage is a fun character and I like him as well, but you can't turn him into Space Jesus!"
Also, I hate what this fic did to Isha, Fulgrim, the Eldar in general (yeah, they get some badass fighty moments, but considering that they're supposed to be fairly cunning and intelligent and able to see into the future, they behave utterly stupidly - for example: what kind of moron would trust Trazyn the Infinite?) Also, I hate this version of the Necrons but I'm not sure the author of The Age of Dusk can be blamed for that - I miss the days when Necrons were near-mindless, implacable horrors, acting in accordance with utterly alien, unknowable goals. It made them different from every other 40K bad guy race. Nowadays, they're cackling cartoonish villains (Trazyn the Infinite reminds me horribly of Skeletor) just like every other Chaos Space Marine special character. Or they're misunderstood noble but ruthless, super-powerful good guys (Szaarekh the Silent King), which I hate because it seems so out of place in the Warhammer 40k setting. But, that's canon. I can't fault the author of The Age of Dusk for trying to work with canon. Even if it is bloody stupid canon. (I just don't like the almost-human version of the Necrons.)
Your mileage may vary. The battle scenes are pretty good (yeah, they're hilariously overblown, silly and faux-heroic, but that's Warhammer 40K for you). I like the multiple Take That's to the worst of Matt Ward's Marty Stu characters. But the good bits are surrounded by so much crap that I just can't stand to continue reading this fic.