So, when last I left off the campaign plan, the PCs will have just finished at the Remorhaz's Rest (aka, the Halfway Inn) and will probably head up the rest of the way to the monastery of Bahamut.
I imagine that at this point the PCs will be cautious about what to expect here, but that's okay, because I'm learning to work around that. Player caution can make for really dull fights. So even though they're being cautious, the environment itself will help spring the ambush I have planned. But I'm getting slightly ahead of myself. The PCs will find that the monastery is a big hulking thing and can guess (correctly) that it used to be a fortress. The large, heavy, and thick gate will be barely still attached to the walls, clearly blasted apart by something powerful. There's a big courtyard beyond and the snow is pretty deep.
That's where the ambush comes into play. Snow covered zombies! Reanimated monks, reanimated minions of the Big Bad, and maybe a wight that's been left there specially for this trap. The first my players will realize anything is still there is when their foot crunches through snow, reaches the bottom, and is grabbed by something waiting there. I'm torn about the snow. I think I want to just make it difficult terrain, but I was at times considering making the PCs slowed for the duration of their time in the snow. Charging, running, or double moving will require an Acrobatics check or the PC instead falls prone. I want a good mix of ranged and melee zombies here, plus a few minions to slow the PCs down.
The second attack will come when the players are inside, probably not in the first room (empty rooms make them nervous), but in the second. And it will come in two parts. The PCs will find more undead waiting for them in the room. No skeletons though. In the second round, more undead will come out of one set of side rooms. In the third round, more will come out of the other set. So the first set of undead have to be pretty tough (or some tough and some sneaky) so that I'm not feeding the subsequent waves into a meat grinder.
The third fight will be the most interesting, because this is where the players will fight the remnants of the pregens that they'd tried defending the monastery with before. They'll be notably warped and changed, the clearest indication yet that some disturbing magic or effect is at work. I want this to be a hard fight because it's the climax (though not the final battle of this chapter of the campaign).
Once the PCs are done fighting, they can try and put together what's happened here. What has actually happened is that the monastery had some bit of lore that the Big Bad needed, which was basically who had been entrusted to guard pieces of the Far Gate and where they were located. That's why the Big Bad attacked Brindol. Just as a distraction from the real target: the monastery. If Brindol had been able to reinforce the monastery, it's possible that it might have been just as potent a fortress as it used to be. The players won't be able to realize this directly, either his objectives or even the fact that he's related to the goblin attacks. But they will be find the library ransacked, some books burned or destroyed, others missing. With enough rolls and checks, they'll be able to put together some names of books, some names of people, and basically get general idea of what they were looking for so that they can continue searching later, elsewhere.
Heading back down the mountain, I want to have 2 encounters. One I've mentioned before, the Winter Court fae attacking the PCs at the bridge. The other one I want at the Dawn site. I want the players to feel reasonably secure, then run into one last trap that the Big Bad left behind, which is some sort of shadowy stalking assassins. I'm going to hint to players the whole way down the mountain that they're being followed and then hit them when they think it's safe. The NPC guide which has been with them all the way up and then most of the way back down? His grisly death will announce to the players that they have trouble. This should be a hard fight too, but not as much so as the one that finishes the monastery.
Then the PCs get the rest of the way down the mountain and back to Brindol, which will finish up this chapter of the campaign. Eight "real" encounters, p[lus the 3 pregen encounters, plus extra experience for quests and the like. That seems pretty reasonable to me for a big chunk of story.