I am now DMing half way through Drakthar's way, and I sense my players feel a real desire to find the rest of the missing abductees if possible, or at least bring justice to those beyond Kazmojen. Pyllrat Shyraat escaped into the underdark and cooler heads prevailed against plunging in behind him after the characters' gather information checks showed Pyllraat has many duergar mercenaries and works for mind flayers.
I had the players find an accounting ledger in Kazmojen's auction hall with info on who the slaves were sold to. Some were sold to the Silent Wolf goblins (to feed to their master Drakthar). Some were sold to "Father," who is a powerful drow wizard that created Kazmojen as an experiment after his troll minions kidnapped Zenith Splintershield's wife. (In my campaign, the kidnapping of his wife by raiders from the Underdark is what helped drive Zenith mad and drove him to his crazy crusade against the Underdark.) Some were sold to Pyllrak Shyraat. And some were sold to Kingfisher (that is how Kazmojen marked down purchases by Vervil Ashmantle).
The party found the note from Vervil (with the kingfisher seal, but no signature) in Kazmojen's vault.
I'm expecting at some point they are going to check out Vervil's messenger business or explore Kingfisher Hollow. While I want Vervil alive for Chapter 8, I'm looking for suggestions on how to handle the party's anticipated investigation into Lord Ashmantle's operations. Ideally, I'd like them to learn something, even if its just a little, to make the later encounter with the slaving trade mastermind more meaningful, yet without getting Vervil or the party killed off one by the other until Chapter 8.
Maybe you could play around with some of the adventures coming up in the series. For example, rather than a Kuo-toa shrine in the fourth adventure in the series, maybe Zenith has recently slain Kazmojen's Drow transmuter "Father" and the PCs have to assault the recently appropriated lair in order to return Zenith. You might have to rework parts of it, replace monsters, etc. but I think it could still work for you. It even makes sense to keep Dhorlot in there as an old ally to "Father" that is working against Zenith and trying to capitalize on the recent upheaval to create more half-dragon spawn.
As for investigating Ashmantle, as a successful and experienced slaver he's bound to have a cover for his slave trafficking. It's either done through the Underdark exclusively or through a proxy business. If it's done with a proxy business, there's bound to be an accountant or someone that is keeping track of the "acquisitions" for him, as well as employees. He's also an accomplished sorcerer, right? So maybe he has developed magical abilities to augment his industry. Maybe in the downtime between adventures the PCs can do legwork and research on Ashmantle, tapping the Striders and other organizations to get information for them, but only making small steps in progress up until maybe a side adventure takes them to Kingfisher Hollow to hault an upcoming auction, where they may encounter Ashmantle briefly before meeting him at the soiree in Oblivion. Of course, there may be clues from freed captives that Vervil took a specific interest in a few slaves, seemingly without good reason. Only later would it be revealed that these were Shackleborn that his masters in the Cagewrights had special plans for.
Good ideas, James. I think I'll try to leave Zenith Trajectory more or less as is, but maybe I'll throw in a few more clues about the drow transmuter who created Kazmojen (in my campaign) in that adventure, or perhaps I'll decide that this being is not a drow, but Dhorlot or Dhorlot's parent, instead, maybe passing itself off as a drow with magic. The mixing of different mix race species and the creation of countless half-dragons does seem go hand in hand. In Bal-Hamatugn the heroes can find more clues about this being. If I keep him as a drow transmuter, then perhaps later I'll either add him as an ally to the Cagewrights (to be fought in Chaper 8 at Thifirane's mansion) or substitute him in place of one of the spellcasters there.
As for Ashmantle, I like the idea of a sidetreck to Kingfisher Hollow which yields only clues, maybe even the identity of Vervil Ashmantle as the mastermind behind the slave trade, but where Ashmantle himself evades direct confrontation until Chapter 8. Vervil does have a cover business, Kingfisher messenger service. I think I'll expand his business interests to also include a transport service for agricultural trade items like bananas, spices, obsidian, etc. secretly transporting and selling occasional slaves along with the cargo. Before Chapter 8 the heroes may be able to expose this illicit business and shut it down, but Vervil himself will stay hidden from them if possible until Chapter 8. He'll be very angry with them for having ruined his business.