Regarding the combat:
I agree with Andre. We actually felt like we had a shot here. The Cavendish showdown and the subsequent run through the ghoul-gauntlet really felt like it was going to have a bleak outcome and at least one, if not several of us were gonna die. That's both good and bad.
On the other hand, I got a sensation that we kinda ran Cevdet over. It's hard to find a middle ground between portraying "Wow, this is going to wipe the floor with us" and "Wow, this fight's pretty easy." The miss chance helped make it a little more difficult, but Cevdet's a wizard and wizards tend not to have much HP, even with assorted buffs.
I think this fight was better overall than the Book 2 final boss battle, but I think there's some room for improvement. I do like how you did the minions, but I don't think the real idea of using minions was really present in the fight.
Using the Cevdet scene as a hypothetical example, here's my suggestion on how to make it better.
Have Cevdet be closer to our level. Seventh or eighth level, max. Keep the krenshar as Cevdet's ally. Keep Nizam as a wild-card that is possible to convert to our side. Then, add a few other creatures around CR 3 or so. Cevdet's a wizard and if he's tamed a krenshar, I'm sure he has a couple of Large monstrous scorpions as pets in his cave.
So then you have:
Cevdet, level 7 Illusionist
Krenshar (CR X, goes down quickly)
Nizam, Wild Card, might switch sides
2 Large Monstrous Scorpions (toned down just slightly to about 25-30 hp)
The scorpions will provide enough of a distraction that you'll have some more time for Cevdet to shine. They're too strong to be ignored, but they're not strong enough to be anything more than a speed bump. That is
good. The purpose of minions and goons is to provide a couple of small speed bumps that don't really endanger the party directly, but still provide enough of a distraction to make the encounter a bit interesting and give your main bad guy a chance.
With this group, we'd probably take out the scorpions in 2-3 hits each, but that's also 2-3 hits per scorpion that's not on Cevdet, giving Cevdet a bit of time to be irritating. I think that would help alleviate the concern you've expressed to me regarding this encounter.
The tl;dr version is: Weaken the main guy, and provide a couple of easy-to-whack-but-still-in-the-way speed bumps to buy a little time.
Regarding the Book:
We've discussed before, but here it is for the record. I like both the mystery and the "railroad" delivery. I think where things fell through here is the clues weren't concrete enough to give us a better sense of direction. So, we floundered around for a while, trying to figure out where to go next, but we didn't have much to go on. It's fine to provide a few different paths that all lead back to the same spot, but I think the clues were a little bit vague. Always remember that the DM will make the connection before the players do, usually. So when giving hints, try to make them things that seem stupidly obvious to you instead of just plainly obvious.
I think the mystery worked, just needs more solid direction
