Truly interesting is the fragile line of events that history takes, and oh how easily it can be changed. Twisting a reality widdershins even a little can change the fate of empires, turn sorrow to joy, or triumph to heartbreak. Sometimes, a universe twisted in such a way may even meet another, nudged just a bit deosil. Those worlds may bump into each other, passing like ships in the night, or they may intertwine so closely that looking back, one would hardly credit them to have once been separate. I own not the worlds I have set spinning, that would be the honor of Games Workshop and Hasbro, respectively.

Our tale begins on the world of Earth, in a surprisingly spartan set of quarters in the highest tower of the largest palace ever built. A man, tall and muscular with long black hair, stern of countenance with high cheekbones and a patrician nose, and the bronze skin of one born to the desert, sits at a writing desk. Those who think they know him would be scandalized that he wears little more than a simple shirt and denim pants of an ancient yet imminently comfortable cut. He raises his hand toward one of the bookshelves in the room and with a miniscule fragment of his immense psychic might, calls a particular book to him.

The book is large, a thick hand-bound hardback whose cover is adorned only with an ancient symbol of magic, well cared for despite bearing an age far more immense than any would guess by judging its cover. As the man opens it and flips through the pages, its contents reveal themselves to be an extensive set of correspondence between two beings, one writing with a firm angular hand, the other with gentle, flowing script. The man’s notes are signed with a name, not his True Name or that which he was born to, but one of many he had borne through the long years - Merlin.

He draws and inks a quill, and on the first blank page of the book, halfway through it, he begins writing.

Dear Princess Celestia,
It has been over a month since my last. My fleet is prepared and tomorrow, I begin the long hunt to find the missing Primarchs. The remembrancers are already calling this undertaking the “Great Crusade”, but I find myself focused more on worries about the well-being of… my sons. It is odd to see my own hand write those words. When I began the Primarch Project, I must confess that I viewed them merely as tools to be used in my grand plan to reunify mankind. My generals and statesmen, my greatest achievement in gene-forging, paramount among humanity, important to my plans to be certain, but until they were taken from me, I had never thought of them as my sons. I do not know if I would have done a good job raising them, and will never have that chance. I thought hard about your advice, and I am resolved that I will do all I can to genuinely be a father to each of them when I reclaim them. They *are* more than just weapons.

Merlin

Within an hour, a new passage appears in the book.

Dear Merlin,
I wish you the best of luck, old friend, and it gladdens my heart to see you write the words that seem so odd to you. I know your universe is far less forgiving than Equestria, or any of the mirror-worlds linked to it, but friendship and family still have great power, even there. Your sons may be vastly different from what you would originally have wanted them to be, but they will still be worthy of a father’s love. Let them be brothers, and encourage them to be friends, and they may yet forge bonds strong enough to stand against the darkness we both fear is coming.
~ Celestia

As the man who was once known as Merlin, now known only as Emperor, waged war across the galaxy to reunite humanity and find his sons, he kept up his correspondence with the confidant known only to a few among his inner circle. Malcador the Sigilite, the Emperor’s seneschal, spymaster, and best friend for millennia, had met her, and greatly approved of her moderating influence on his friend’s at times terrifyingly cold nature. Without Celestia of Equestria’s advice, things may have gone much differently, from the order of the acquisition of his sons to the way in which he handled some important issues. Some key missives hold clues to what was and what could have been.

Dear Princess Celestia,
This week I found another of my sons. He was named Angron and used as a gladiator by the people of a planet called Nuceria. To say that life was unkind to him where he emerged from the Warp would be an understatement of the gravest kind. His brain had been implanted with foul augmetics that greatly increase his aggression and prevent him from knowing a moment’s peace. I found him the night before the slave revolt he lead would have made its last stand. He refused to come with me. I was half-tempted to take him without his leave. I didn’t have the logistics to evacuate his people in time, and I couldn’t just obliterate the butchers he faced, for his brother Roboute had already brought them to compliance. I agonized over the decision for a few minutes until Malcador brought me to my senses and helped me realize the obvious. I must admit I felt rather stupid for feeling that much angst when you’ve probably already noticed that the solution was built into the problem. When the forces of Nuceria’s rulers massed for their final attack on Angron’s rebellion, I led my Custodes in a teleport deployment between their forces, introduced myself openly, and demanded they stand down. There was some… initial disagreement, but I made my point and they obeyed. I had my ships land and with the time restriction and enemy army no longer factors in the matter, I was able to evacuate the rebels. Angron wept, something I get the feeling he very rarely does, and thanked me for being the first authority figure in his life who actually cared. He is still damaged, but for the first time in his life, he has hope. It is an amazing feeling to have brought that to him.

Merlin

Dear Princess Celestia,
I finally had a talk with Lorgar about his fanaticism. Thank you for talking me out of making some ill-advised grand example of things. I went against my instincts and decided to let him know the truth of some important things. He finally, truly understood my distrust of religion, but he argued that if the horrors in the Warp were such terrible things, then maybe all of the devotion he brought forth and directed toward me would help stand as a bulwark against the Dark Gods. It is an interesting position, and not entirely without merit. He does also at least now understand that he should focus more on actually making progress than spending inordinate amounts of time making sure every world he takes is fanatically loyal before he moves on.

Merlin

Dear Princess Celestia,

Barely two thirds of my sons have been found, and I have lost two of them to tragedy. The XI Legion is no more, and its Primarch slain at great cost. The II Legion is mostly intact, but without its Primarch, I have folded them into the Thirteenth. Out of grief and shame I have commanded that the names of the fallen be stricken from Imperial records, and I shall not name them here. The Primarch of the Eleventh was a paragon of nobility and virtue who became corrupted by an ancient xenotech device that turned him into an inhuman monster. He bound his legion to him through some horrific method granted to him by that device, and they were slain to the last. Those of my sons that I had found already fought alongside me in facing him. The Primarch of the Second was a skillful orator and negotiator who had won over many worlds with words alone, though he was willing to back up those words with force, and yet he was broken and turned by the Eleventh. There was no way to save him, and Russ ended his suffering. The Eleventh was felled by Guilliman and Horus standing together, but he rose again, and Magnus and I were forced to use our powers to obliterate him, body and soul.

Merlin

Dear Merlin,
I know only a shadow of the grief that weighs on your heart now. And I can only offer you the advice that was given me almost one thousand years ago when I was forced to banish my sister. Reach out to others. Do not bear this burden alone, and remember that there are those who truly care about you for your own sake.
~Celestia

Dear Princess Celestia,
Thank you. Your advice, and Malcador’s, helped a great deal. Roboute also helped. He is among the most human and empathetic of my sons in temperament and is both observant and stubborn. I also had a chance to talk with Magnus about his delvings into the Warp, and their dangers. I’m not sure he took it to heart like Lorgar did, but he has resolved to be more cautious. I must prepare for departure soon. I’ve glimpsed the thread of fate that will direct me to another of my sons.

Merlin

Dear Merlin,
I have just encountered the most magically gifted unicorn I have ever met. A young filly who just earned her cutie mark, a symbol I know very well indeed. And the burst of magic that earned her the mark was triggered by something even I have only seen once before, a sonic rainboom. I felt fate itself intertwining the destinies of several ponies into that event. She, and the friends she will have in time, have a great destiny ahead of them. I have taken her under my wing, as my personal protege. All of my plans and gambits are nearing completion. Fate willing, when my sister returns, the Elements of Harmony will be taken up by ponies bound by true friendship. When I forced the Elements to my will to stop Nightmare Moon, I could only draw upon their power, and could not truly use them, for Luna was the rightful bearer of half of them. Perhaps those ponies bound to the fate of my new student, Twilight Sparkle, will be able to do what I could not. I can scarcely bring myself to hope that things will play out as I desperately wish. I am not sure my heart could take it if this last chance to free my sister fails.
~ Celestia

Dear Merlin,
My faithful student just attempted to warn me about the return of Nightmare Moon. She was worked up enough that she didn’t realize just how quickly I would have had to write my reply if I hadn’t already prepared it months ago. She is en route to Ponyville, on the edge of the Everfree Forest, ostensibly to oversee the finishing touches for tomorrow’s Summer Sun Celebration. In truth, the ponies she will meet in the process are far more important than the details of the celebration. The stage is set. The curtains are rising. In the next two days, I will finally know if all this has been worth it.
~ Celestia

Dear Merlin,
THEY DID IT! My sister is returned to me, free of Nightmare Moon’s influence. Oh how I have missed her. I have given Twilight Sparkle the task of learning more of friendship, and I know that she and her new friends will have the fate of Equestria in their hooves more than once in the future. Who knows? They may even be able to provide a more satisfactory solution to Discord than leaving him encased in stone for eternity.
~ Celestia

Dear Princess Celestia,
I am gladdened to know that your sister is free of that foul corruption and stands at your side once more. I have to admit I rather liked her. I have found another of my sons, who has called himself Konrad Curze. He wasn’t in the best of circumstances before I found him. In point of fact, he had taken control of his crime-ridden homeworld by brutally murdering anyone who harmed another. I’m keeping a close watch on him, and have left a garrison force from the XIII Legion on his homeworld of Nostromo to ensure they do not fall back to their old ways now that they no longer have their terror of the “Night Haunter” to keep them in line. Guilliman opted for about half of that force to be remnants of the II Legion. A force of idealists might be just what the world needs to grow beyond what they were.

Merlin

The correspondence continued apace, through events great and small, Celestia having someone with whom to discuss her grand plans and the manipulations necessary for them, and Merlin having someone to help keep him grounded and in touch with the best of his nature. Due to the strange difference in the flow of time between their realities, the decades stretching into centuries of the Great Crusade passed as the weeks stretching into years of Celestia’s plan to prepare Twilight Sparkle for her ascension. The Primarchs were found and worlds brought into compliance, as Twilight became Princess of Friendship and in time took two students of her own, one with whom she communicated through a book very like the one whose extracts we have been perusing.

One day, a message came that Celestia had long been half-expecting, for she knew her human friend as well as any could claim to.

Dear Princess Celestia,
I have fought alongside all of my sons, and by now, I feel that I should get started on my project to give humanity access to the Eldar Webway. We have fought a great battle on Ullanor and I am set to name Horus as my Warmaster and have him take up leadership of the Great Crusade. However, I feel that not all of my sons are ready for me to run off for what many of them would see as a glorified science project. Aside from those of my sons with problems of their own, such as Angron and Konrad, there are a number of rivalries that divide them. Russ and the Lion fight honor duels every time they meet due to a brawl they ended up in over what was known millennia ago as “kill-stealing”. Perturabo and Dorn resent each other for their matched specialties. Guilliman, for all his empathy and humanity, has a bad habit of wording important things he’s trying to tell his brothers badly enough to anger them. Mortarion despises Magnus for his psychic might and keeps trying to get me to ban all psykers from the Legions due to his own prejudice against them. I would like to ask a favor of you. Could you spare your protege and her friends as consultants to help sort this out? By my reckoning of the current time difference between our realities, a week in yours should translate to a couple of months here. I believe that trick you described Twilight using to reach the mirror-world using the link between Sunset’s book and hers would work between ours. It would take me less than an hour to rig a proper portal on my side.

Merlin

Dear Merlin,
I have asked Twilight, and she is willing. I have given a rough outline of your reality and the situation, but you will have to fill her in on most of the details. And I concur regarding the transportation arrangements. It would be best if they didn’t have to deal with full armies of your Marines, so bringing together all of the remaining Primarchs in a controlled environment would probably be for the best. Have them each bring one member of their respective Legions as, I believe you’d called them, equerries? It would give the twins an excuse to both be present without spoiling their secret, at least.
~Celestia

Dear Princess Celestia,
Good idea on the Equerries. I’ve gathered my sons together on a defense station above Ullanor, with a company of the Adeptus Custodes as an honor guard. I’ve finished the portal and am ready to receive those you are sending to me. Thank you, old friend.

Merlin

And so the stage is set and the players ready for our tale.

Author's Note

Here's a new story I've been working on for a while, born out of a number of odd mental images such as Konrad Curze looking uncomfortable while being enthusiastically hugged by Pinkie Pie.

'Tis largely a fix-fic intended to alter the circumstances that in canon lead to the Horus Heresy, through judicious application of ponies.

Editing this to include an omake, showing the particulars of the "initial disagreement" on Nuceria:

The copper-haired giant stands defiantly between his rebels and their oppressors, face already contorting into a rictus of pure hatred. He beats his axes together in a steadily rising rhythm, taken up by his forces, and he howls. Righteous anger, seething hatred, defiance, and pure, unreasoning rage blend into a terrifying, primal sound. To the shock of all present, just as that terrible roar reaches its crescendo, a deafening staccato of thunder and lightning tears along the ground between the two armies. Angron’s vision clears first, and he sees the man he met last night, his father, standing tall in golden powered armour that is as much an intricate work of art as it is protection, a flaming sword in one hand. Standing with him are figures almost as tall as Angron himself, clad in slightly less ornate armour. Most bear glaives with bolters built into them, while those closest to the Emperor bear broad-bladed swords, themselves grafted with bolt pistols, and large shields worked with the Emperor’s two-headed eagle crest, the aquila.

Angron watches as his father walks to the leader of all Nuceria and stands before him, disapproval radiating from him. “I am the Emperor of Mankind, to whom you have sworn allegiance. The army you seek to destroy is lead by one of my sons, whom I am here to reclaim. Commendably, he refuses to abandon his men. I am working on something of a deadline here, so I will give you one chance. Stand. Down.”

The last two words hit home with a palpable force, but the man who has hunted these “eaters of cities” for so long flushes red with rage. “This is an internal matter. They are rebels and murderers, and you have no jurisdict--” His tirade is cut short by the precise sweeping arc of the Emperor’s sword, and the flaming halves of his corpse fall to the ground.

“Wrong answer. Does anyone else seek to challenge my authority?”