Disclaimer: I do not own Harry Potter and Warhammer 40K.

Shout Out: Honestly, I blame Fallen-Ryu (Marriage Law Rejection Letters (Letter 64) founded by Sakura Lisel) and DeusExTranshuman (Get Me Out Of This Hell Hole!) for this one. I knew of Warhammer universe in the passing, but reading the works of those two ignited my own merry amusement binge, and here we are. Warhammer universe is one of the strangest to comprehend because it is so vast and truly keeping the characters as they are, is a massive uptake if you don't read original works. I don't have originals, but I still hope that I did their voices justice in this little snippet.

I'd love to see more Harry Potter/Warhammer crossovers if anyone is feeling adventurous enough to fill that particular section of the fandom.

I was also due writing a Hogwarts Rejection Letter as it were, so...

(The letter is intentionally in sections with their authors written along because all of the Primarchs wrote literal essays as to why they do not allow Harry to learn Hogwarts' substandard education.)

Synopsis: Harry has 19 big brothers that absolutely refuse to comply with Wizarding World Shitfest aka Hogwarts School Invitation. Big Daddy is also Not Amused.

Hogwarts School of Witchraft and Wizardry,

We, the Primarchs, are writing to your ... esteemed institution of learning in regards to your letter to one Harry James Potter, offering him a place with you to learn your kind of psykery - or, as you call it, magic.

You are offering to school him for seven years before he would be legible to come back to us - never mind your promise of 'school holidays'. Where we are, such kind of thing doesn't exist, so excuse us to being dubious to your ...offer as it were.

And we say no to you taking away our brother. We've been torn apart for far too long to just allow your kind to take our youngest away from us, never mind your... reasons. We had to wait 300 years for him to come back to us and we are lucky - extremely so - to have him with us again. With him, we are complete, he is our light in the darkness we were created to war against ever since Emperor gathered us together to realize His plans in order to defeat Ruinous Powers - /Sanguinius/

/

You are NOT taking away our youngest brat, do you hear me? Sanguinius may be too bloody polite to get to the gist of it, but as it is, we, Horus and Angron will fraking burn the world around your ears if you – /Horus and Angron/

/

I will teach him. The kid expressed interest of learning art and languages with me, and he has a whole lot of ideas for architectural designs –/Perturabo/

/

Little cub is ours. He may still be a small pup, but his teeth are sharp and I will sharpen them even further. He already has a pair of Thunderwolf cubs in his pack, and I almost busted my gut with laughter when feathers nearly got a heart attack when Harry was apparently adopted by the trio of carnodons when he had been thrown into the pit. Best day of my life, really. And I wouldn't get between Perturabo and his teaching ambitions, either – /Leman Russ/

/

Ferrus will skin you alive if you try to take the runt away and I will help. Actually no, I will skin you alive, he can just roast your worthless carcasses over fire – /Jaghatai Khan/

/

Excuse Khan, he was always one of the more bloodthirsty ones of our lot. But I can not allow you to take him away from us, not with so many things at stake here. I See what you intend to do, and I warn you - Do. Not. – /Konrad Curze/

/

Curze may be a fear-loving moron, however I agree with him. I cannot, in good faith, agree to send our youngest brother off to your institution to be enslaved to your practices – and for seven years, no less – /Corvus Corax/

/

We do not approve of your high-handed invitation. Do not force us to employ extreme measures to deal with your foolhardiness - /Alpharius Omegon/

/

I do believe all of my sons would outright revolt if I even tried to entertain that far-fetched suggestion of yours to send their youngest brother to your so-called school. And I know for a fact that Ferrus Manus, Fulgrum, Vulkan, Rogal Dorn and Roboute Guilliman are currently holding down Magnus, Lion El'Jonson, Mortarion and Lorgar in order for them not to storm through the Warp straight to your planet and completely decimate it, never mind the dangers that such travel would have brought to their very minds, bodies, sanity and souls. It is hard enough to hold back Sanguinius, who is admittedly the mildest-tempered among them, Harry excluded, but you just had to ruse Horus and Angron, didn't you? They are currently baying for your very blood, and as level-headed Roboute Guilliman and Ferrus Manus are, the one thing they are all united on, is their brother's well-being.

Never mind Harry's own legion. Death Wings may be the smallest among their brethren legions, but what they lack in their size, they more than compensate in their ferocity and devotion to their Primarch. (The daemons outright flee in terror whenever they see them on the battlefield, which is a feat none of other legions managed to replicate as of yet, though I do think that Sanguinius' crowd is the closest to such an achievement, but they would still need half a century to be properly versed in daemon crowd control to the degree of Harry's own bunch of menaces...)

I am very tempted to allow them such an indulgence. I personally owe my youngest son a great debt of honor and life, as do the rest of my Primarchs. Incidentally, Sanguinius and Konrad Curze have Seen what you intend to do to our youngest, Albus Too Many Names, and I. Do. Not. Approve. My son is not a sacrifical lamb for your interests, however well-intentioned they may seem to be. You have led your people to this ruinous path they are forced to tread now, and in my eyes, you are no better than Chaos Gods what with your ambitions and meddling in the fates of many. It does not befell to my son to correct the mistakes that you yourself have made due to the actions and inactions you have uptaken in your mortal life.

Keep insisting that he has to, and I promise you that mankind as a whole will be relieved of having to deal with festering wound that are your people. Permanently.

Regards,

The Emperor of Mankind

Afterword:

Harry is a son of the Emperor of Mankind himself, and instead of being thrown on a planet, the Chaos forces have warped him to the side universe in order to hide him from the Emperor's sight. Considering he has protected his siblings, Harry had arrived to the planet very weakened, and Voldemort's attack didn't help the matters, though it did have one positive side effect - it opened the psychic link to his brother Sanguinius. His body development followed to that of normal humans because most of his energy was diverted to upkeep of the psychic link and powers branching into magic territory.

Harry stops Horus Heresy in its tracks, due to his psychic link to Sanguinius and forced Apparation to his side, whatever ritual had Lily used to protect him plus Harry's own power, drives the taint of Ruinous Powers away from Horus and other affected people. However Ruinos Powers do have the last laugh - Harry is killed by Horus in their last ditch of an effort, and Sanguinius lost his shit tus promptly kicked the shit out of Horus who also regained his senses due to Harry's intervention, but too little, too late -

Emperor stopped Sanguinius from outright killing Horus what with calling to him that the boy is alive. (Snake-Voldie in the scar died, Harry got a blink to the Other Side before returning back to life due to Emperor's calling and Death's blessings as its champion.) Admittedly, even Emperor himself was afraid of Sanguinius in a towering rage like he had been back then.

As it were, Harry chose to stay with Sanguinius, choosing not to return back to Dursleys, much to Emperor's silent relief and Sanguinius' joy. Harry charms the other Primarchs too, nearly giving them heart attacks what with adopting – or being adopted by the most dangerous and bloodthirsty animals ever (hello, charcodons here,) and the said animals are intelligent enough to use speech. Mechanicus also love the boy to bits because of his quirk to repair machines and if they could get away with it, they would've kidnapped him for themselves (Note: Tried and failed.). Sanguinius has weakness for the boy preening his feathers.

Primarchs as a whole care more for the runt, as they affectionately call him than their father, and it does help that Harry just accepts them as they are, and they learn to care for him, so they are not awkward beans with zero acceptable social interactions in society as a whole. Chaos Forces are in for an interesting times (more like whole lot of whump on their side, but that's neither here nor there).