This chapter describes what happens if partpy accept Vizeran's invitation to go to his tower.
PCs can also try to sneak into the tower. I would not recommend it. It's a magic tower of a Drow Archmage. TPK is very likely.
The six floor tower, named Araj (Drow name "vengeance"), is inside a big cave. There PCs will meet also Vizeran's apprentice, a youn a cynical Drow mage.
They'll learn something more about Vizeran's past and they'll realize he's urge for Revenge against Gromph Baenre, a personale nemy of his since childhood, and Menzoberranzan. However his desire to banish Demons into the Abyss is genuine too.
He shares his plan to PCs: call all Demons where they were summoned, Menzoberranzan, and let their nature work; they'll start fight betweeen each other and when one's corporal form is killed its essence will eventually return into the Abyss to regenerate itself. When the last one will stand, presumibly weakened and wounded by previous fights, the PCs will kill it and send it into the Abyss.
For Demons' calling he needs to craft a magical device, Dark Heart, and it takes to look throughout the Underdark to find the components needed to craft it.
Vizeran offers to the party his arcana knowledges to craft the Dark Heart and to use it at the right moment, while PCs must find some of the components and kill the last Demon.
PCs may have doubts to forge an alliance with a wicked and ruthless character like Vizeran. They can cfart and use the item by themselves if a Wizard is in the party or is an ally of them.
You'll see in the following posts which are the components and where to find them.