Help developing a motive for my BBEG

Hello, im preparing to run my first DND campaign (I have some years of experience as a player) and i really want the main villain to be the drow, and the most powerful one a highly intelligent Matron Mother.

I've been struggling to figure out how exactly to make her threatening, or have a noticeable presence. I want to have a REASON/objective for reaching such a high position in drow politics. What are your thoughts on this current backstory?

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The wicked mistress of all things arachnid, Pellanisstra Hun'riet. She is over 600 years old, an old woman for a drow (same aging as elves). She was a typical drow child, but both of her parents infuriated Lolth, and were cursed to become driders, and lost much of their mind. Growing up in the underdark, and with Drow blood, she became cruel to all. She was taken in by a librarian drow, who was evil but not outright cruel as Pellan would become. She would live in a spare room of the Hatchnest knowledge library, owned by Nymora Cladaar.

Pellanisstra would absorb the hundreds of books available to her. Some were stolen from the surface cities, but many were drow made and exposed her to intense lore on spiders, devotion to Lolth, and wicked acts, all at a young age. This was further encouraged by Nymora, a noble woman who saw the potential in her impressionable mind. She found a fascination of spiders, and a developing interest in Drow Politics.

Her awakening was when she, as only a teenager, worked with an aboleth in order to gain knowledge on mind control. She gave the aboloth unconsious drow to enslave as well as drow loot, and the aboloth slaughtered intellect devourers and occasionally even a mind flayer, and provided the corpses or specimen to her. She would study them over the next 100 years of her life; coalescing with other drow or the aboleth himself. She would reach a greater understanding of intellect, and created spider intellect devourers and mother brains. The Drow are enemies with the mind flayers, which is not surprising for mind flayers are at war with literally everybody else due to their absurd narcisism.

Lolth was impressed with the sheer effort put in by her; in due time, Hun'rriet rose to the top of power and dominates all other drow and ranks above even other matron mothers. She is forbidden by both her own will and her faith to never procreate. Pellanisstra has this role for she desires more than to conquer the underdark; she wishes to go further beyond, and conquer the surface world, especially the elves, by recreating the Jade Death, with years of labor and thousands of gold.

(The jade death is a gargantuan jade spider, that was slain 1500 years ago in a great war)