Context:
I introduced my 12 yr old daughter and her 3 school friends to "DnD" by running a small dungeon crawl for them. To my delight, all four of them caught the bug and I ended up running them up to level 6 over the course of 8 sessions. I wrote my own adventure path, taking parts and pieces from old DnD 1.0 modules as well as Pathfinder modules, and weaving my own story for the group. It's been a lot of fun, but also challenging because this groups of kids will not stay on any rails - I have to build the sandbox and with several plot points and let them weave through them in the order and style they wish.
In our last adventure I had to throw them a way out so they wouldn't all die, and off the top of my head I had a group of Driders come come out of a side passage and team up with them to take out a small horde of undead. The session concluded with them heading off with the Driders to meet the local lord.
They are 1-2 sessions max away from completing the Adventure Path (the end goal of closing a portal door).
At this point I want to give them a memorable event as they close out the adventure.
And I think I might have accidentally created the basis for that when I added in the Driders without much thought. Obviously, the Driders invitation to meet the lord cannot be with good intent. The party is already a bit suspicious but it was the end of the session and they lazily agreed to be escorted. (I thought they would run/escape).
So . . . looking for ideas here - even seeds. I would like to sow paranoia or give them a tough choice that will have hard consequences either way. Maybe play the Driders against the local Drow. Maybe they have to make a choice between two really great choices. What would a great escape look like?
They have to exit the mountain in a manner that allows them to reach a hidden valley (where they have an encounter that concludes the Adventure Path).
I'm interested in the whole setting. Why do they have to close the portal? Is it a portal to the underground which continuously pumps out undead, and they're sent to stop it?
I like morally ambigious choices myself, and I've had my players been forced to step over the corpses of innocent a couple of times.
There was a campaign where they were stuck in a vampire-ruled city with everyone living in fear of their nocturnal lord and his followers. They had to gather relics to be able to fight him, and were forced to make some tough choices during the way.
It all ended with them being forced to kill the lord who were sacrificing everything he had to save his subjects in memory of his dead wife.
He had purposely made himself the antagonist of his city to direct all anger and hatred towards him in a "an enemy's enemy is my friend" kind of way.
He tried to catch my players (all with high bounties on them) to gather up enough money to avert the future starvation of the whole city. My players left the city knowing they they had chosen themselves over the lifes of thousands.
Of course, you might want a little brighter ending to your daughter's campaign. But not all choices are correct, and sometimes you make the wrong choice knowingly.
The portal is in the process of being opened and it is the means for a large, defeated army (now undead) to return and challenge the King again. Various groups have made it through the portal to scout and recruit.
They have an object (a necklace) that was part of earlier loot that is the key to closing the portal. It has many similarities to the portal so once they recognize that they should realize that destroying the necklace destroys the portal (the necklace has many types of resistance so destroying it will require some other items they have accrued).
So far that's not very memorable though. Maybe one of them has to go into the portal and close it from the inside? That player would think they are making a sacrifice, but then once the portal closes I would spit them out somewhere, safe and sound.
Yeah, the fake sacrifice is a good ending, which will leave one with the sense of doing the right thing. But they will have to be engaged in the story to make it really memorable.
Is the undead army inherently evil, or would you like to add a dash of gray to the usually black-and-white?
I think you should definitely focus on a "good ending", but I wouldn't rule out the bad ones from happening. I think it's important that they feel that the ending is dependant on their actions. Of course, the ultimatum you described is a good indicator.
Edit: If you want a plot twist you could switch the challenging side. It could be known that the King is looking to extend his land, and is secretly plotting to have them fail their quest so that he can invade through the portal (he could be an ignorant old fool, if the undead forces are that much stronger. Or maybe he just wants his name to go down in history as the king who suppresed the undead).
Who knows, maybe he has made a deal with the lord of the driders, who will now try to kill them? (The reason the driders helped them would be because they don't know how the portal is closed, and aim to silence the group after they've gained that knowledge). This particular plot twist (where the king is trying to assassinate them) only works if it is known that they are trying to close the portal.