[X] "I couldn't ask you to do that." Despite it all, it was better to learn of the Emperor's horrific secrets than live in ignorance.
[X] Write-in: when speaking to Magnus, "Look, I understand why you are terrified of revealing this to Mortarion, I am tempted to let you wipe my memory for it...but remember how you felt when you confirmed about the Archdaemons from me and how much it hurt that I kept it from you? Imagine how it will gut Mortarion when he finds out you kept this from him just after you and he were starting mend bridges, as I can only imagine that happened when he mentioned going to you for aid.
Keeping secrets like this, from each other, will only tear each other apart. After my service in the Maelstrom, the emperor gave me Boons...one of the ones I asked him for was the right to tell all my brothers, including you, the truth about what is in the Warp and the Archdaemons therein. In that spirit of trust, fraterinity, and understanding, I ask you to work with me on this and follow my lead. Especially as there is an Archdaemon that delights in precisely this level of infighting and deceit.
So please come with me and tell Mortarion, if he hears it from me and only me, it will not only lessen the credibility of what we are trying to say but will make it that much harder not to lose another brother. We have to do this now, while the wounds are still fresh, otherwise they will scar and it will only make things worse. Between myself and Horus, and you coming through, we will be able to temper Mortarion's wrath and get to the bottom of this, but it won't work if you don't come with me. In a very real sense, you are now in a position to determine the fate of the Imperium in no small sense. I understand your fear...but if we hesitate now, the ones that hurt your legion will win and revel in the pain and suffering that result. So come with me brother, trust in me, trust in mortarion and horus...but most of all trust in yourself, that you will be able to along with the rest of us, see this through."
Before we got to Mortarion, Kesar gives him a big, heartfelt hug and says only four words: "I believe in you." and turns around, trusting Magnus to walk with him to speak to Mortarion.
To Mortarion, "I have finished conversing with Magnus and while I wanted to wait for more of our brothers to join us, Lion's presence pushes me to reveal it sooner...In light of my service in the Maelstrom, the Emperor granted me full authority and right to reveal the secrets that he has been keeping about the Warp and the beings within it. I say this because what Magnus found when he was divining is not only in line with one of the most powerful beings would show in order to create strife and discord between us...but it is a being that revels in magic, and so would be able to create illusions or false divinings. However, as without this knowledge, Magnus only saw visions of something so horrific, so wrong that he wanted to gather more evidence and prove it before bringing it to us, however we arrived too early and thus caught him before further investigation. Magnus was in short caught between sharing nothing and being thought a fool, or revealing what he could only suspect and thus cause a near civil war.
Brothers, there is something so wrong and flawed on this planet that we all felt it, psyker or no. We need to take anything we cannot prove or have verified beyond means of divination with a grain of salt. I do not know if the beings of chaos are behind this...but the battlecry of Angron's legion, "Blood for the Primarch, skulls for the twelve legion...that is near word for word the battlecry of one of the forces of the four most strongest beings in the Warp, the same ones that Horus came face to face in a recent journey in the warp. Mortarion, I ask that you forgive Magnus for wanting to show the prudence we all have been demanding of him and seeking to establish solid evidence of what he Saw. After hearing him, I can understand why he was terrified of sharing.
In summation, there is much we do not know for sure and we have to verify it more in depth as there are two terrible scenarios I see here: either Chaos created this situation in order to tear the Imperium apart, or the Emperor actually did this of his own free will and is thus clearly compromised, as best as we can tell one of these is true, and we must find out which. There are other scenarios of a more mixed nature but those require even more evidence and only prove my point.
As we do, I will share what I have learned about the Archdaemons and their forces so that you too can understand why I am concerned they may have been an influence here. And yes Horus, before you say anything, i am fully aware that I might be wrong and that the presence of 'Chaos' as it is referred to might not be here...but that is why I plead with you all that we must focus on finding the solid evidence of what has happened here. Even if they were not, there is something in this world that will draw them if they haven't shown already."