In this chapter PCs go to an Underdark's area known as Labyrinth, an abandoned huge mine full.
Vizeran has ordered party to look there for the fabled Gallery of Angels, a cave where they can find the six angel's feathers needed for the ritual.
As usual you won't find a map of the area but there's a new random encounter table.
The Labyrinth is very large and it's easy to get lost. For the Wisdom (Survival) check give an even higher DC than suggested.
The Labyrinth is home for two groups: one of minotaurs under the influence of the Demon Baphomet and one of gnolls, frenzied by Yeenoghu.
Party can have some encounters, nothing special. The real one interesting is with some Modrons who can guide PCs at the center of the Labyrinth, where they'll find a strange machine, the Maze Engine (read below).
They'll witness Yeenoghu itself with some gnoll minions fighting with a goristro and killing it. They'll can grab other two components then: some drops of Demon's blood and the heart of a goristro.
Make PCs target of Yeenoghu's fury even in the following chapters. It's a perfect evil opponent: violent, bloodthirsty, furious and with many minions ready to fight against them.
The Gallery of Angels is a good test to let PCs try something different: puzzles, unusual situations and random madness will be the rule there but with Tymora's blessing they'll grab the 6 feathers.
As already written, in the Labyrinth party can stumble upon an artifact called Maze Engine, whose existence may be already known if they did speak withBeholder's prisoners in the previous chapter.
The Maze Engine is controlled by Baphomet, who has left there some minor demons guarding it.
PCs can defeat them and try to activate it but it's not necessary to complete the adventure.
The Maze Engine is the typical artifact which causes chaos and random events. It can be funny to let players toying with it but you should be aware you can have a TPK.
Having found the components PCs will be ready to enter in the mighty PG and dangerous Menzoberranzan to find the last object needed for the ritual.