The shadow brought into the light is a coordinated attack by all factions before they all joined the Imperium completely.
The attack was carried out after the Dark Eldar repeatedly assaulted countless sectors and either taking multiple cities or destroying countless people.
I am unable to find any clear information about this war. but from what I have been able to discover it was a war, led by Isha herself and it was one where, Human, Eldar, servant aliens, and even Necrons were involved, all of them agreeing on one thing and one thing only.
Kill the Dark Eldar.
Ther are even reports that demoness, the servants of she who thirsts were involved.
The war is said to have been so massive and even some say that it is still going on.
Yet no one that is stationed at the edge of the webway portals dares to enter them further than they already have, is a simple reason, one I will now say.
The web way is collapsing.
However, there is another reason why they don't enter it and it is the same reason why Guy remains alive.
When Isha finally managed to force the Drak Eldar back into the web way, she ordered guy to do the unthinkable.
She ordered him to use her authority and sever the connection between the web way lines that have been discovered by the uneasy alliance and connect them to the one controlled by the one that screams in the dark.
It should be noted that when Guy performed this act, the glow of the Astronomicon shined even brighter than ever before, of course, there is no way that these two events could be connected.
Regardless of this as a direct result of using so much power Guy was rendered unconscious for several months.
Its this event that finally caused the Sisters of Silence to be deployed and capture him, however, a few months before they could encounter Guy, he would wake up and participate in the war that forced him to revive Ferrus Manus.
Guy resurrecting both Primarchs, caused both Isha and Arturia to demand they should be allowed to return to his side, even though both women would be returning to what can only be called jail.
The sisters of silence themselves found themselves being met by the penal legions, both the humans and the Astartes.
The fact that Sanguineous was standing next to Guy when they demanded he accompanies them, didn't help their cause as every weapon was brought against them.
Tensions had grown to the point where the sisters themselves acknowledge that had they started a fight, they would have lost on all fronts, as for what they have discovered everyone that has dealt with Unit before, is incredibly protective of each planet they have lived in for an extended time.
Tensions only calmed when both Isa and Arturia arrived and calmed everyone, a few hours later one of the Custodies arrived and said to everyone present that the Emperor wanted to see Unit and all of its members.
It should be noted that Magnus the Red showed a great deal of interest in Guy and both Rogal Dorn and Ferrus Manus, threatened the Red Primarch if he attempted to make contact with the man.
"That may be true but they didn't say I couldn't come here and see all of your records"
"Lord Magnus"
"Keep doing your job record keeper, I just want to see everything this man has done and I have already begun to ask myself several things."
"See here, there is no way a Human could survive this and yet the situation changed in the most desperate of times"
"...Yes, that's it..."
"Would you like to know something interesting. A light creates a shadow, so multiple lights create multiple shadows, but father as well as my brothers and myself, aren't what you would call normal"
"All of those shadows meet at a certain point and concentrate the light they capture giving it form."
"That man is that light, the question is, is he the combined shadow of all of us together? or is he someone else entirely?"