A number of people have commented that the PCs are unlikely to see module #6 coming, as the clues are subtle.
I like to do foreshadowing as a GM, and I have a player who is *all over* subtle clues and proactively pursues them. As a result, the PCs have known about Nyrissa in some detail since module 2. We are now at the transition between modules 3 and 4, and their awareness of Nyrissa is coloring everything they do.
At first I thought this had been a terrible mistake, because Nyrissa is an awful thing for low-level PCs to contemplate dealing with. The player and I did some brainstorming, and decided that we needed (a) a timeline for Nyrissa's attack, so they'd know it wasn't tomorrow, (b) concrete things to do to prepare for it early on, (c) a setup in which they knew for a fact Nyrissa couldn't physically come after them, nor they after her, until her appointed time.
Once we had those in place--Nyrissa will show up in 6.5 years, and can act only through intermediates until then--the player knowledge started to be a big asset. Without it, the PCs would be working on conquering Brevoy, which is not the game I wanted to run. With it, the PCs will put off that plan until post-Nyrissa, which suits me just fine. The threat of Nyrissa is giving focus and context to a lot of their kingdom-building and alliance-building efforts. I invented some ancient Nyrissa-stopping events, and the PCs are exploring to find relics of that period; this has helped keep the exploration game (which peters out after module 2, in my opinion) fresh and purposeful.
There's always a fear that knowing about the BBG too early will lead either to a catastrophic failure of PC morale (this happened to us in _Shackled City_) or a disastrously premature attack (this kept trying to happen in _Rise of the Runelords_). But if Nyrissa is not accessible until "the stars are right", the PCs can focus on hounding out her minions (hello, Irovetti!) and getting their kingdom shipshape in the meantime. And by the time they face her, she will not be a nameless foe; they'll know a lot about her and that should make the final conflict more meaningful.
One thing I will definitely add is an attempt to sway the PC King the way she went after other local male leaders. It's inexplicable why she doesn't do this (except that it is hard to handle in a module for unknown PCs) and I think I can get some good drama out of it. It won't be Nyrissa in the flesh, of course, as she can't in our setup: but Nyrissa in dreams or by proxy is already quite a visitor.
Overall, I think it works to have the PCs know, at least if there is a clear date. It would have been hard to set that date in the modules because kingdoms develop at different rates, but by somewhere around Varnhold the GM should know about this and be able to set the date appropriately.
If your players have a detective/planning orientation, as mine does, I recommend this route.
How did they find out about Nyrissa by the second part of the AP?
Also, you mentioned Irrovetti. Do they already know he's her pawn?
I'm running Kingmaker, and my PCs are at the end of book 6 now - they didn't learn the details about Nyrissa until they got their hands on Briar.
They felt that there was someone / something moving against them, but not knowing who / what it was made things tense. They thought Irrovetti was the culprit, but then realized there was a bigger player. At one point, they wondered if it was another kingdom.
I feel that keeping the BBG hidden makes for more drama in this AP, but as long as you're having fun, you're doing it right.
I've decided to seed knowledge of Nyrissa into my game through a piece of rather odd treasure - the life-sized statue of the elf woman in the Fey castle in RRR. She's actually a victim of a flesh to stone spell, and knows much about Nyrissa as well. I'm hoping that the PCs will eventually realize that the sculpture is a little too perfect, and perhaps seek out a stone to flesh spell to repair the damage, at which point they'll find a very disoriented elven princess who can give them some details earlier on.
Of course, I'm reworking much of Nyrissa and the late-game stuff anyway, so it's a different approach, but I think it's a nice flourish.
Well, I am keeping it to little hints for now. The dead Unicorn, the Stag Lord having a ring made out of green nymph hair and silver, and so on. We'll see if they make a connection if those nymph hair items keep turning up.
What Magnuskn wrote!
And I gave them a vision/dream as early as in book #1. They were on the verge of taking the stag lord's fort when one of my players had other more pressing matters on his agenda and I wanted to discourage my players from trying it with only three heros (not because they would not have been able to, but out of the feeling, that the final encounter should be an experience shared by everyone).
So, on a really stormy night, while they were huddling in their dismal cave north of the stag lord's fort, they had fog "attacking" their campsite coming at them like waves crushing on a beach.
Out of the fog they heard a hunting horn and finally a vision similar to the wild hunt appeared (he serves as some kind of messanger for the fey in my game).
The hunter threw an iron cauldron at them (which has ties to running Falcon Hollow as a prequel to KM for my group) and I had Nyrissa rising out of its depths dragging one of their former companions with her, flinging the corpse in their midst, laughing maniacally at them and then the vision/dream faded.
A failed Will save with a fair DC made them frightened and they left the area.
Similar things have happened since.
Also, the dead body of the ranger in KM #2 had a similar to the stag lord's ring.
Also it is mentioned that Nyrissa is collecting trophies from the material world - I will expand on that theme, too, and leave traces of her passing now and then.
Ruyan.
Where did they find the clues? Let me see if I can reconstruct:
They questioned everyone who had worked for the Stag Lord in detail, and given how alcoholic he was, I assumed he'd let a few things slip over the years. (Why have all that backstory if it is impossible to discover?) So they knew a mysterious woman had motivated him.
The core PCs are Irissen (it's a long story) and in order to give them more initial ties to the area I had them following in the footsteps of a lost Irrisen expedition/colony in this area. So they were collecting clues to the fate of that expedition, which was entrapped by Nyrissa. I put the lost Irrisens on Candlemere Island, in a magical sleep behind an impassable barrier. They didn't get through that until very recently, but it definitely made them think.
They did *not* find the ring--they hit the owlbear before I was ready for those events to happen.
They found a crude image of Nyrissa among the trolls' belongings, as a hint that the strange ambitiousness of the troll king was also prompted by her.
They found the dead unicorn and asked everyone they could about it. I had the local faeries be very evasive, which caught their attention. (I realized later that Nyrissa did not kill him, as she is, in my timeline, not able to act in Golarion yet. Luckily this didn't lead to a contradiction.)
They caught the Pitaxan agitator, and for reasons I personally think were ill-founded decided that this must mean Irovetti was a pawn of "mystery enemy". (Sometimes PCs put together more hints than really go together, and I think this was one of those.)
They researched the history of the area. If Nyrissa is a major reason that it has never been successfully settled, its history must reflect that.
They defeated an undead hag tied to the cult of Gyronna, and she mocked them with Nyrissa's return.
Finally, the player forthrightly said that it would suit the game better to have more information, as he knew enough to be paranoid but not enough to be useful. So I trickled parts of the story out through various human, centaur and faerie sages. The faeries must know; and this is a kingdom with faeries (Falchos) on its Council. In fact, it is a kingdom that will tithe six girls every summer to the faeries of the field (though it wants them back in the fall)--they are faerie-lovers to a fault. (Non-humans in general, actually. The General is a winter wolf, the King is a hobgoblin, and no one is quite sure what the chief druid is.)
A lot of it was, I think, a reaction to the high encounter/event density in the first two modules. All this stuff keeps hitting us! Why? If you poke around on that question you start to end up with Nyrissa, especially if you add in a few that aren't really her but could be, as my player did.
Overall I'm pretty happy with this. I didn't need her to be a surprise. The game's about building a kingdom; the PCs should get to build toward their climatic challenge.