Once I realized that the Sanctum of the Serpent Vaults was going to be a trilogy of adventures, I realized that the Citadel was going to be my Castle Ravenloft. It was inevitable. I had already established the Tyrant as a potential villain and this would be a sort of payout for that.
The trick was I didn't want to make the same mistakes that the various incarnations of the Ravenloft adventures made. All try to force the party into the villains story and agenda in various ways.
While their agenda might be interesting, save that shit for your unfinished novel...
A gaming adventure is about the people at the table and their agenda.
But a good villain needs to have motivations and hooks. I needed to think of a new method to do that without creating a set path. I had also set up the possibility that the party might even be on the Tyrant's side through the faction system.
They might be a sympathetic character by the time the party reached this point and the Necrolord, Hierophant, and Chancellor could be the real problems they need to solve.
Means I need to be flexible. So what was the answer?

Doom of course. My all time favorite tragic villain. The one whom, when well written, like by Johnathan Hickman, carries weight. He is driven. He will do evil. He will do the right thing and be honorable. His word will not be broken. He loves and is fragile because of it.
Honestly, he is an awesome gothic villain and a way better template than Strahd. He also does one, very important thing that makes all this possible.
He monologues.
What if my adventure text was filled with the villain's monologues? Each room a flickering, brief hologram of him speaking something relevant about that room and about them. Dropping a detail about the complexity of their character the party and GM can choose to use how they want...
What if that is coupled with a random encounter engine designed to illustrate the chaos of the many-sided civil war that is happening in the halls of the Citadel?
Well, what happened is I wrote a first draft really quickly. Now I am refining each chamber before turning my eye to finishing up the art and the stats for the various potential challenges along the way.
In the meantime, here is the unedited draft of the opening monologue of the Tyrant. I hope you like it as much as I liked writing it.