This was an...............interesting read.
It breaks canon with other lore, and I don't think these are spoilers since everyone is/should be familiar with this.
In the audio drama 'Malcador' (ya know, the one with the cover art with Malcador sitting on the secondary throne with the armrests decorated with the 'II' and 'XI' skulls, but which were never touched upon at all in the story), and which I believe was a short story in one of the many compilation HH books: Malcador is leading an elite human guard officer through the Imperial Dungeon, and strange noises, roars, barks, etc demon noises can be heard from behind a ginormous door. Malcador vaguely references that He is fighting the battle for the very fate of humanity. There is reference to Custodians and Sisters being tensely at the ready on our side of the door, should things go bad. We know, as readers, that the Emperor is personally battling chaos forces within the webway portal to the Throne Room that Magnus ripped open. And, at some point, Malcador sits the throne in place of the Big E while He goes off to do something (IIRC, it is this battle for the webway where Malcador takes over, but that has been retconned a few times now). In this book, The E of M, instead of Malcador taking over, the Big E has the Silent Sisterhood capture psykers everywhere, and invokes the first use of the black ships in order to power the throne while the Emperor is AFK. Curiously, as we know from lore, the throne takes 1,000 psykers a day to keep powered, but in this book they make it clear it can 'seat' up to 10,000 psykers. For some unknown reason, they only use 1,000, even though it's obvious they could have easily captured more psykers and plugged them in to the throne.

Ok, possible spoilers follow******************************************** ************************************************** ********************

There is (and I most likely reading in to this WAY too much) some key references to Ian Watson's trilogy, where the Emperor's consciousness is so overwhelmingly powerful, that it breaks in to shards and speaks to the Custodians at different points in the Emperor's lifespan while everything else is happening in the present. We see the Big E enthroned to deal with 'Magnus's Folly', and the Custodians, mechanicus, Sisters, and possibly Imp Guard are deployed to a heretofore unknown and long dead Eldar city inside the webway to defend it against the impending traitor and chaos forces intent on using Magnus's arrogance as a means to take Terra. We also learn of Drach'nyen's origin, which apparently is stronger than Chaos altogether (there's a really weird scene where the Emperor allows himself to be impaled by Drach's talons (double parens, oh my! some serious foreshadowing to His fight with Horus) and then the Emp absorbs Drach'nyen, turns it into a sword, and impales one of His own Custodians with said sword, because the Emp can't destroy this one particular demon, even though the four main Chaos powers quail in fear of The Emperor. Meanwhile, besides this one demon, the Emperor's will channeled through His sword is not only killing but completely unmaking HORDES of demons in every swipe of the blade).

We do learn the Emp really does NOT give a $#!t about the Primarchs, but only sees them as tools to an end. This is literally stated on page 211. He further drives home that the Primarchs were full of arrogance and hubris, which may have been a mistake. So consumed with their own glory, that He remarks only three of them questioned why The Emperor decided to reveal Himself when He did (we know this as The Unification Wars, or thereabouts). He doesn't even refer to them by name, but only as their numerical identifiers, and barely treats the rebellion of Horus with any gravity. If anything, He cares more for his Custodians, which He addresses by name and honestly cares for when they die. It is also carved in stone that there's 10,000 of the Custodians, but barely any of them left after the battle in the webway. The Custodians are also detailed as being closer to Primarchs than Astartes. They wear armor made of 'auramite' instead of 'ceramite', and can keep fighting even with half of their brain missing. Hell, there's one scene where the main Custodian is fighting traitor World Eaters en masse, and slaughtering them, and he is outright putting them on blast as weaklings, shouting that their Primarch was the only one who failed, that he needed to be rescued, he let himself become a slave where all the other Primarchs became kings and nation builders.
There is, despite what sounds like a negative review from my perspective, a well written explanation as to why the Emperor did not see the rebellion coming between pages 212 and 215.

So, this is where the retconning happens: once the Sisterhood shows up with 1,000 psyker happy meals, Malcador is nowhere to be seen (or even mentioned with any substance in the entire book), nor does Malcador take the Throne while the Emperor goes off to finish the battle in the webway (which, in the original lore, is how and when Malcador dies).

There is an unexpected twist reason as to why the Custodian's include a broken legionary in their ranks, but this is worth reading on your own, and I won't spoil it here.

While it was cool to see the Custodians front and center finally, AND to see the Big E in action, it didn't really feel like it furthered the story at all, nor did it keep with well know lore all that well. While A-a-ron Dembski's writing style is normally fantastic and engrossing, it was very cut and dry, as if he had GW Iron Warrior Overseers looking over his shoulder to approve every word he wrote. The Custodians are cardboard cutouts, with little personality, and they are soooooper arrogant d-bags, outright belittling 'lesser' Astartes warriors.