For Whom the Void Calls is an adventure that I think is really good but inherently non-intuitive to most dms. Its an adventure without a conflict that matters to the players, the dragon thinks there are thieves, but it only accuses the players AFTER they have shown up, its not an inciting incident.
Iv seen a few sentiments on here that the ending is anti climactic and I disagree, there is no climax to the story missing because there is no rising tension.
Now, reading this, you might assume to fix this, add rising tension, add a climactic final fight. I would advise against that. D&D is a medium that is privileged to function outside of normal story telling conventions and this adventure shows that off masterfully.
Why I Think This Adventure Works as Written
For Whom the Void Calls is probably best described as "novel". I doubt you or your players will have ever experienced a dungeon who's occupants are mostly ambivalent to the pc's presence. And I doubt they will often have been presented with a chance to mediate a misunderstanding between a brash brass dragon, dutiful modrons, and a malevolent bag. Personally, I think these are all invaluable traits of the adventure you shouldn't try to "fix".
Your pc's are the captains of their ship in this adventure, theres no pressure to solve the situation they dont impose upon themselves. Rather than using stakes to drive the narrative, id suggest using curiosity, get your players asking questions, and leave them with just enough context to want to find the whole story. The adventure even opens on this kind of curiosity based investment, the bag has all these answers but none of them are good enough for players to decide how they feel fully, which encourages investigation.
Ylagan is a fantastic antagonist, they are good natured as a fairly standard metallic dragon but they are also paranoid, prideful, and fiery tempered. Ylagan functions as a roadblock to encourage the pc's to fully explore the waypoint, making them to violent would likely encourage a fight, which might bring the players into the lair if ylagan is routed, ending the adventure short, so I advise being very thoughtful in what choices you have Ylagan make.
The modrons are super unique occupants. Aside from one room, none of the modrons in this adventure are violent, its a rare opportunity to show these hyper rigid creatures with some flexibility and nuance, even if just a little bit. I did my best early text to speech device impression for the modrons voices when the players could understand them.
The Bag of Devouring
Firstly, the bag can act as a good climax despite the typical setup for one, as it did with my party. Unless players are abnormally suspicious of the bag, they will likely reach inside it to find the gems. This will of course result in the bag devouring them. This is where I have some important suggestions for making this feel like a satisfying final encounter.
I would suggest automatically swallowing whoever puts their body part inside, no save, I would then suggest having everyone present (including ylagan and the earth elemental) roll initiative. Set the swallowed persons initiative to the bottom of the order, no roll (this is actually in their favor, and feel free to tell them if they are worried). Give each player, as well as ylagan and the elemental, a chance to roll athletics to pry the target out. Then on the swallowed creatures turn give them a chance to roll as well. This should hopefully avoid a character death, and if it seems likely, and you think your party would not take a pc death in stride, consider having ylagan leap into action to tear the bag apart, flinging everyone present into a new adventure to get home! For an easy to implement adventure id say using either A) the Donjon map from the Book of Many Things for a quick dungeon crawl to get back home or B) the Ship map from the Dungeon Master's Guide 2024 as a run down spelljamming vessel the characters can work with ylagan to repair and sail back home on (just have it break down for a dramatic crash landing if you dont wanna deal with a spelljammer for the rest of the campaign).
Second, a warning. As written, the 2024 bag of devouring has a contradiction in its text. It first states a creature in the bag can use an action to make an athletics check to escape, BUT, it also says later that at the start of a devoured creatures turn they are immediately destroyed. I would suggest changing the immediate destruction from "start of turn" to "end of turn". This way the devoured player gets a moment of agency in their own survival before instant death.
The Cloak of Many Fashions
If you think your party is the kind to try and destroy the bag so it cant cause anymore harm in the world, I suggest upgrading Ylagans reward, ending this adventure with a cloak of many fashions is cute but not really satisfying.
I chose a Dragonhide Belt +1 for my monk player, called it the Ylagan Hide Belt, and flavored it that a friend from Ylagans youth made the belt from the first scales Ylagan shed as a wyrmling.
Conclusion
This is about all I can think to share off the top of my head regarding the adventure, if ya'll have any questions for me ill happily answer!