So yeah....chaos magic to make female space marines....who were breeding tanks....
Honsou, You Sick Fuck
We know that you among the fans, despite all our attempts to dissaude you, want cute FEMALE SPHESS MAHREENS! And there is good news - technically, there ARE Female Space Marines IN OFFICIAL LORE. The catch? Yes there is a catch. And this one is the worst.
Your FSM is a bloated, twisted, undying, insane, mutated pregnant mother-slave only alive because she's in the Eye of Terror.
You see, Honsou of the Iron Warriors, in an effort to squeeze new MAHREENS into the ranks without having to pay that other sick fuck Fabius Bile for clones, created a system of hideousness and sadness that pushes grimdark to a whole new level. He invented the Daemonculaba, a system whereby a woman is chained and fed stuff which will morph her and her womb into something that was never meant to be. A boy is then (yes, a child), jammed, shoved or cut INTO the vagina and stays there for a while. When his time is ready, he, now XBAWKS HUEG and STRONG, will have to rip, tear, or otherwise push his way out of his adoptive mother's womb/vagina. When he emerges, he will either be a skinless mutant abomination, or will emerge as a fully functional (albeit skinless) Chaos Space Marine in a fraction of the time, with all his growth stages and bionics and gene-seed already taken care of. How? It's fucking science.
This goes back to our Female Space Marine - you see, while she was being morphed and changed, Dark Mechanicus Biologos hereteks worked on her body, implanting gene-seed and making her physically a Female Space Marine, even if the form is rather off. Thus, her womb can turn children as well into Space Marines by having her womb 'grow' the required organs and stages directly into their bodies in a fraction of the time, days and weeks instead of years. Thankfully for the entire universe, this horror was destroyed before Honsou realized he could stick girls in there as well, and get double the recruits, and stumble upon the secret to true Female Space Marines.