The End and the Death is a hard one to review, and I've been mulling it over for a while now. I've already typed up multiple essay-length musings on the third part of the novel over at Bolter & Chainsword, and putting them together into a coherent whole would.... take a while.
That being said, I consider part three easily the strongest out of the trio. It's tighter, doesn't shift focus away from the events that really matter, that really excite, nearly as much as the previous two sections, and various scenes had surprisingly large emotional oomph.
However, it also cements the problems of the first two parts (and by extension Saturnine by being riddled with continuity problems; these start in the very first chapter/interlude of the book, get worse in the second, and the entirety of the climactic duel between Horus and the Emperor hinges on Abnett ignoring previously established plot points, character development and character arcs, particularly with regards to Horus in Wolfsbane/Titandeath/Slaves to Darkness.
In many places, Abnett made the characters fit the story he wanted to tell, discarding what aspects (a word he was particularly fond of this time) he didn't need or like, and at times dialing characters back to previous states (effectively to back when he last wrote them), without much if any regard to the intervening events that shaped them in other novels.
More glaringly, perhaps, is the way the book ends, or doesn't really end, its plotlines. Some of them simply serve as elaborate setup for the Abnettverse Inquisition climax Pandaemonium, to be picked up again at the author's convenience. Others get glossed over entirely - for instance, the retribution fleet never makes planetfall, the Khan never wakes up, Katsuhiro - one of the Siege-exclusive point of view characters of the first half of the series - gets to speak in one brief scene, completely replacable in all his mentions throughout the trio of books, never contributing anything of worth.
Meanwhile, we are being clobbered about the head with repetitions, both in terms of prose as in dialogue & catchphrases. Valdor especially got it rough, his speech often boiled down to a "damn you"-curse, where before he had been one of the most interesting characters around whenever he received the spotlight. His characterization directly contradicts other books he appeared in, as well - which is even more troubling as it looks like Dan isn't done with the character yet.
If I had to be uncharitable, I'd call many of the things that happen in volume three contrived, arbitrary and on the nose. References and winks at the audience are common - so much so that they get tiring, frustrating. If I told you not one but two characters from the same previously established group end up being supplanted by fakeouts before the book is done, you'd probably think the Alpha Legion is at their A game - but you'd be wrong. All that setup for them we had in volume two? Went nowhere, really. No, this is down to switcheroos.
Another switcheroo: Remember Ollie Piers, the conveniently named guardsman from Saturnine, who would lay the foundation for the myth of Ollanius the Pious in the final battle, by amping up a real event until it was myth-like, and that being the whole point of an entire major plotline in said Saturnine? Yeah, it's obsolete. Abnett pretty much made his own prior creation, which was already too on the nose, and made it redundant.
There's a lot of stuff like this throughout volume 3, and by extension the entirety of The End and the Death. Unearned character moments, moments that can only occur because the author-that-be can put the characters exactly where they need to be at any given moment, be that in the action or fridged in a desert for hundreds of pages.
Did I enjoy my time with the book? Yes, sort of, but only when I put aside the series' legacy of, what, around 64 prior works that made and shaped many of the characters used here. The climactic duel had impact, shock value and was far better handled than I would have expected going in. However, it also hinges on said contrivances, so when the initial "wow"-momentum faded, the aftertaste started feeling all the more bitter.
It's certainly a better book than the previous two parts, but even if I was to put them all together and examine them as a single work, the way the author asks the reader to, there'd be numerous discrepancies, uneven plotlines, loose ends and miscelaneous problems with the novel. For a book that tries to rival War and Peace in length, it's just too inconsistent and irreverent of other authors' character developments for me to be happy with.