This is just a theory (well, two stuck together with duck tape) and I go a little off topic at times, but hey, head canons all round. re reading chapter 11 would be recommended.
We don't know much about Kiel's mother other then what Keil tell's us, she did not worship demons, but believed that they would come back one way or another, and was trying to figure out how to deal with the inevitable, looking to open multiple gates to find the types of demons- the sentient and the animalistic.
I believe at one point, she was working with Shad, Khaless, and maybe even Shasana trying to learn more about demons
however, Ven left fI don't even want to try speculating on her past before that, lesser noble, commoner- all that matters is that one point she was maybe teaching at the school and was lab partners with the sharen sisters.
Why would Shad go to someone the rest of the underworld believes to be insane, if she didn't know that she was good at what she did? Shad personally visited Ven with her DAUGHTER (in chapter 11 that woman standing smug next to her is the earliest form of Kalki as I recall), so maybe they considered each other close friends.
Ven's complete shut down of any offers, and consideration of writing to the other clans about Shad and her plans might have been about manipulating seeds to do a wide array of things, such as imploding in twenty years, causing infertility, all the fun stuff we've already seen, but she needed Ven's help, trying to lure her in with titles, a position, whatever that would get Ven to come over and help her on her project, recruited those that she could trust to get the job done. When that tanked, she sent the Sharen squad after all of them and only through Kharla did they even survive. In comes Sene'kha.
Sene'kha who from what I see was Ven's right hand woman- she was there when the discussion took place about the seeds, and her rebuttal. Sene from what I recall was already planning to take out Ven for leadership, having sought out Kharla- when Kharla was found to be so powerful and maybe the early Vloz's only defense against further attacks. Ven continued to refuse to make Kharla the puppet leader, so Sene had Kharla kill Venn.
Sene then gave the information required to Shad and the Sharen backed off and started to go ahead with her own plans of summoning the demon god. While Ven was not a demon worshiper, some might have been drawn to the Volz believing they were and I believe Sene was one of those people.
I also believe that Ven was literally in the room and she was the one that summoned the demon!Khaless in the first place, destroyed the portal herself then left due to the monstrous figure- maybe the demon fresh out of it's world swore vengeance on the Elf races from Sharess's corruption of their world, which made Venn believe that the demons would come back one way or another and set off to work herself.
So Shad offered Ven a position, a new job and wanted the information on the nether gate that Ven used to get the Khaless!Demon to taint herself with it, as Shad is untainted in chapter 11. The demon might have become Khaless as it's default form and taken her personality and Shad believed that tainting herself with a Khaless!Demon and then assuming control with the merge taint that they recently made- it's still a large debate on whether Shad is Shad or a demon that acts like Shad, or a combination or what. She might have been manipulated, convinced by Khaless!Demon by telling her about her type of demon and Shad came up with the plan to concur the underworld.
Then I'm moving on to Kiel and her very unique taint, which seems the opposite of Khaless's abilities where Kiel takes in lost demons and makes them a part of her instead of assimulating them completely and not made into shoes she could put on and off is she wishes- if indeed that little blob is Naal- but only of they want to.
This seems to vary on the personality of the person, but since there are only three with the dlack demons, I can only same my data samples are too small. I'm assuming that these black demons are easy to manipulate to their host. Khaless had shapeshifting before her accident and her merged powers allow her to literally consume the life of people and take their faces. Since she's the only example I have of that, it might be that black demons are 'hives' or sorts, and Shad is a 'queen', where instead of taking in demons, she can make seeds from herself.
Not that she's tainting others with her own seed in the beginning, no, she lets them suffer with a different taint and when she helps them through the merge, personally, she separated apart of herself into that person and assimilated the weaker demon and drow, and makes them an extension of herself, and explains why she stopped merging after a couple, not only to keep the safe taint in high demand, but because she might have needed a break from spliting parts of herself off.
How easily these demons change form might be why they were affected by Sharess so much, she manipulated them with her own aura and changed then from what they once were, whatever that was or destroyed their food supply of other demons.
I'm not sure how Keil got tainted, maybe it will even be explained in the next few pages of this chapter (or not), but it might be connected to her mother's past and research. I'm coming up dry for this one.
On a last note: Ven's actually pretty old- Ven has her natural hair colour, and with Kiel's just as vibrant hair I'm assuming that Ven was second generation and Keil is third, explaining how she would be lab partners with the twins. on the flip, Sene's actually YOUNGER than I first thought, in chapter 11, you can barely see her ear's through her hair, while Ven's is pretty long, she might have been in her 30's 50's at the time of the takeover.
Chapter 11 is one of the oldest chapters in the comic that are considered still canon, due to it not having a rewrite i'm assuming all is canon at the time of writing even if it's a little iffy.
TLDR:, Venn had a hand in bringing the black demons into the drow world and the black demons excel in multiplying and assimilating other demons into themselves. how their powers work depends on the drow themself or how much they are merged together and the relationship they share.
Daydream version: Sexy crazy demon scientists destroy the underworld by experimenting with nethergates to much.
what am i even doing i've sent an hour on this post it's 3 am whaaaaaaaaaaat let me sleeeeeeep why did this even become a thing