I feel it necessary to lay out some foundations for the AU before continuing. I won't reveal everything here as I plan on doing that in the story itself, but it helps to know some of the major changes from the original timeline (OTL) and even from the Nobledark Imperium AU (which you can read on 1d4chan).
The basic premise is that the "sane" races have allied together in a grand alliance in a united front against Chaos, Necrons, Tyranids, Orks, etc. This alliance includes the Imperium, the Eldar, the Tau, the Kyn, and additional minor xenos races. For lack of a better word, people are generally more willing to work together (IE the Imperium is a lot less xenophobic and the Eldar are a lot less arrogant).
However just because the "good guys" have more cooperation with one another doesn't mean the galaxy is safe. Chaos is a lot more powerful because both the Emperor and Isha are alive and active in the galaxy (more on that in a second). While the Horus Heresy didn't happen, plenty of Marines and Traitors have fallen to Chaos over the years. In addition Croneworld/Chaos Eldar exist, vestiges of the depraved old Eldar Empire who have aligned themselves with Chaos, as do Ashen Kyn/Chaos Votann, those leagues and clans who fell during the Long Night and now are a dark mirror of their kin.
The Necrons have woken up faster and the Silent King has significantly more direct rule and control over the dynasties. The Orks sometimes align with Chaos, knowingly or not, and are becoming ever scarier and more organized as "Brain Boyz '' appear in their ranks meaning that figures like Gazghull are a lot more common and capable of unifying Orks in larger Waaaghs and empires. In fact, the War of the Beast takes the place of the Horus Heresy in this AU with its destruction and character-establishing moments taking place here. The Tyranids have also arrived earlier, and now their main fleets are showing up far in advance of their OTL progress.
As for the Emperor and Isha: Early on in this timeline, the Imperium made contact with the Eldar who agreed to a frankly insane plan to raid Nurgle's Garden to rescue Isha. After it succeeded, the two races have only been working closer and closer ever since. Also in this timeline, the Emperor isn't a mortal or a being made of fused shaman souls; he is one of the last Men of Gold, a highly advanced, physically powerful artificial human-made during the peak of the Great Human Dominion before the Long Night. Spared its horror as he was in a stasis pod that shielded him from the insanity to cause the Men of Gold and the Men of Iron to turn on humanity. The Emperor has gone by many titles such as the Warlord, the Attendant, the Steward, and finally the Emperor.
The Emperor and Isha (also known informally as the Empress) are actually married, their union symbolizing the union of their people as well. What started as a purely political move has turned into a more genuine relationship and the two often form the "Travelling Court" that visits the worlds of the Imperium to resolve issues, offer advice, and hear grievances as well as lend their considerable martial aid. They are worshiped as living gods, mostly by their respective peoples, lending the Alliance a much stronger metaphysical and spiritual bulwark against Chaos, especially combined with the fact knowledge of Chaos (and more importantly how to safeguard against it) is more commonly known. However, the Imperial Court cannot be everywhere at once and the gaze of the Chaos gods focuses a lot more on realspace in this AU because of it.
Whereas in canon, the grimdark nature means that things suck and you can't really change things, the nobledark nature of this AU and its original AU means that while things still suck, people are at least well-intentioned and well-meaning for the most part. The galaxy is dark and dangerous and full of horrors, but you will always be able to find a friend and ally to hold the torch against it.
An example of the difference in attitude between this AU and canon is how the Astronomicon functions. In canon, a thousand psykers are literally fed into it a day just to keep it running with the desiccated remains of the God Emperor atop it, even as its operations are barely understood, an unliving monument to the atrophy of humanity. In the Brightest Night, the Astronomicon is run by the Adeptus Astronomicon who have a huge choir of specially trained pyskers to power it after the Eldar help to restore and upgrade some of its systems. The burden however burns out the eyes of the choir members and they will eventually die from the strain on their bodies. But each one of them is given a burial with full honors and their names inscribed on a massive golden stele in the Imperial Palace. The Imperium is not built upon Human suffering , it is built on Human sacrifice and cooperation .
There will likely be more questions that might not be directly answered by the story. This page might see edits to further explain things. Or I might just make an entirely separate story dedicated to explaining the AU.
