Honestly, I've waited too long to do this, and my memories of individual incidents in the last couple of sessions have faded to uselessness. There is something I'd like to bitch talk about, though.
This "sandbox"? Not so much. I've mentioned this before, but it continues to be an issue. In each of the first three books there has been a problem--Stag Lord, Trolls, Vordekai--which the PCs feel pressure to expunge. They are aware of the problem, if not from the very first page, then shortly after the adventure begins. What in the world is supposed to stop them from heading out to take care of the issue as soon as they discover its whereabouts? What in character, I should ask. Sure, the players probably have an idea that this is the boss battle and they need another level before they head that way. But IC there is no reason they should not charge off to do battle with the foe.
Let's take Varnhold, for example. (Reiterating cut-text spoiler alert, but I'll reveal nothing here that will be a problem in my game.) We begin with a vanished town population. Regardless of the hooks used to draw the PCs in, they should end up in short order at Varnhold. Unless they are evil or neutral with evil tendencies, they are probably going to skip hexploration en route; innocent lives could be in danger. Once they arrive in Varnhold, the clues they come across should confirm that the townsfolk are probably in need of a rescue. Even if they might suppose that most or all of the people are dead, they still (again assuming good alignments) should feel it necessary to make sure. So, still no deliberate hexploration and likely a trip to visit the centaurs. Once they get there, given the information that the centaurs will share if made at all friendly, there should be a sense of urgency to go to the Valley of the Dead. Whether the townsfolk are alive or dead, whether the disappeared centaur has shared their fate, the PCs should by this time be aware of some details about Big Evil. So long as the possibility exists that anyone is still alive, there is no reason for the party not to make a beeline in that direction.
Sure, a few Neutral Practical-aligned parties might take some time, presuming that if the townsfolk are still alive by now, they will probably be alive next week as well. But most Good parties are going to have at least a couple of characters who will advocate for immediate investigation, because no one knows how long any theoretical captives might last, and besides, it would be cruel to leave them there for a moment longer. And regardless of living captives or no, Big Evil may be getting stronger and is certainly still active. Everyone in the Nomen Heights could be in mortal danger.
Thing is, they are not ready--not even close--to face the Boss Level in this book yet. All of those arguments are perfectly logical and likely in keeping with IC ideals. But out of character, I have watched the players struggling three times now with the OOC knowledge that they are probably talking about the biggest, baddest encounter in the book, and that they will almost certainly die if they try to take it on before gaining no small amount of xp. There is no sandbox here. The players must hexplore through all or most of the "random" encounters before facing off with the final set piece encounter. To make it an actual sandbox, each encounter would need to have two or three scaled levels, so that it did not matter when the PCs headed in that direction. But even ignoring the idea of the sandbox, this is poor design. If the story requires meta-thinking to keep the party alive, something is wrong. (Not that there can't be meta-thought, only that requiring it to make the story work is absurd.)
I will need to read more carefully in the fourth book and try to circumvent this if it happens again. (I was taken a bit by surprise and had not reread the centaurs section before I had to run it.) The most likely solution is just that the information needs to be delivered to the PCs later or more vaguely. The biggest problem with Varnhold Vanishing is that they go to the centaurs and the centaurs say, "Yeah, we know where Ancient Evil sleeps. It's right over those hills..." Simplest fix then would be to have them know folk tales about Vordekai and for perhaps older centaurs to recall stories from their elders about a time when watching the Valley of the Dead was a duty, but to have the actual details of both Vordekai and of the location lost decades or even centuries ago. Then the group must explore the Tors. In doing that, I would probably also have moved the location further south, so that the party didn't stumble on it as soon as they hit the mountains.
That's all for now. Game is tomorrow. We'll see if characters win out over players and I end up inflicting a TPK.