When you consume the vial containing the essence, a portal from a chaotic hellscape will appear either immediately or after some time. 18 capsules will drop out from this portal which will immediately close itself before any Daemons could pass through.
*Upon inspecting the pods which have suddenly opened, you noticed that each pod has a single baby in it (except for maybe one, which may have twins). You are now the adopted mother/father of the 18 (19?) canon Primarchs of Warhammer 40k, and it is your job to take care of all of them.
*You have excellent parenting skills, allowing you to handle 18 (19?) children without neglecting any one of them, and can shower them with parental love and affection without showing any favoritism.
*Even with limited resources such as money and food, you can still properly take care of the 18 (19?) Primarch children without straining yourself.
*By constantly being in touch with your children and having them interact with each other as a family, they will never grow up with the flaws they had in canon Warhammer 40k such as Lion El'Jonson being a loner, Konrad Curze growing up as insane serial killer, and Angron being angry all the damn time, they will trust and help one another albeit with some friendly sibling rivalry.
*The Primarchs will display some behaviors that they would have gotten if they were in their original home planets, such as Konrad Curze fighting crime, Vulkan's preference for fire and forges, and Lorgar's need to worship a deity (he will try to worship and venerate you if you are a being of great power).
*You can properly convey what your intentions and feelings towards your children are without any miscommunication and blunder. Whether they accept your reasoning is up to them, though you are incredible at convincing your children as long as you take your time.
*If you properly took your time in taking great care of the Primarchs, they will see you as their true father/mother instead of the Emperor and will show absolute loyalty to you as long as you don't constantly mistreat them.
*If you want, the Primarchs can be from the If the Emperor Had a Text-To-Speech Device universe, where they will have their parody counterpart's personality. Such as Magnus being a comic book nerd, Jaghatai Khan being a speed freak, Lemon Russ being a furry, and Corvus Corax being an emo.
*After carrying out the babies out of the pod, you noticed that each pod has papers that have information on how to create each of the Primarch's legions, equipment and other essential items. For some reason, females can't be Space Marines and the only reason as to why as you read through the papers was that "The Adeptus Astartes can only contain males. No Girls allowed. Girls are Yucky".
*You can decide to be in Warhammer 40k for this though you will face off against legions and legions of Daemon trying to kill you and the Primarchs, the asshole xenos such as the Elders and the Dark Elders trying to kill you all, and eventually the Emperor of Mankind himself. That's not even getting into Warp bullshit like Magnus talking to Tzeentch, Curze's frequent nightmares, and the warp trying to do its hardest to tempt the Primarchs to their side. So you probably shouldn't do that unless you really want a challenge.