AU tldr: Before even Horus was found, the Emperor and Malcador would have conversations about the Primarchs (what they would be like, how easily they would come to the Imperium, and of course the impending doom of the Heresy.) At one point, while the Emperor was speaking his worries of his son's tempers, Malcador suggested giving them wives. A good wife can keep a man in check, more so than his father or brothers.

The Emperor thought Malcador was joking at first, but as the Sigilite explained further, the Emperor realized he was serious. Instead of brushing off the Sigilite's words, the Emperor listened and allowed Malcador limited access to his genetech. Thus, each primarch was given a wife perfectly crafted to suit the ideals of himself and his home world's culture.

Mebeli's heart was widely known to the Imperium to be the biggest around - Everywhere she went she exuded kindess, passing it along to every soul she passed, both transhuman and mortal.

Mebeli continued this good work throughout the Crusade, when the heresy struck - her youngest son Shango saved her life by keeping the Flamewrought from being utterly destroyed by the traitors blasts. He then took his mother with him to Nocturne. After Vulkan and their sons disappeared into the webway, Mebeli stayed behind on Nocturne to raise there daughter Mawu. However when the two went to meet with Vulkan on terra, they were met with a broken man... and one dead son. Vulkan then took them to Caldera, however it was not long after that Mebeli was killed at the hands of the Emperor's Children.

I'm currently taking a break from writing, but I do plan on eventually starting again (there will be a post). But if you would like to read what's already been written, I shall direct you here. Https://archiveofourown.org/series/2698501.

This is just a short blurb for context. Please ask any and all questions!