Heyo fellow WotC forum-goers. I'm trying to finish a multi-part adventure for my group that could potentially change the face completely of one of the areas of my homebrew campaign world. As such, I want the climax to be epic and contain as few holes as possible. I have the advantage of knowing they are in the thick of it and will want to put an end to the trouble plagueing said area as soon as possible. Here's what I've got so far, feel free, in fact, PLEASE DO critique the outline and any potential holes in the story. Tell me if I add something that would demotivate you from continuing if you were a player!
I'm going for a mystery that eventually culminates in the players questioning where their own loyalties really lie, and acting upon those loyalties to end the chaos in this kingdom once and for all.
I'll update regularly with work I've done on it as I have a month to prepare for this and hopefully I provide you with something interesting to read while you provide me with feedback I need to make this a near perfect adventure!
Background
The lonely kingdom of Gaverdon lies on the western most tip of the largest continent in the world. It now finds itself seperated from the mainland after a cataclysmic event that caused a legion of devils to march forth from the nine hells, razing everything in their path. The people were saved, the city, however, was not. Led from the city by their king before the great magics that would banish the devils from the land destroyed it, they founded a new home in a valley that lies a safe distance from the old capital and what lies within it.
Now, 10 years have since passed and so has their heroic king, finally slain by a wound that
could not be cured by mortal hands. He leaves his kingdom, and his legacy, to his sole heir, his 21 year old daughter, who would become the youngest individual to ever inherit the throne. True to her father's ideals, but lacking in ability, she strives to rebuild the kingdom of her forebears no matter the cost to herself. There are those among the nobility, however, who are unsatisfied with her. Those who believe that they should have been the ones to lead. Accusing the queen of not being a legitimate heir to the throne on seemingly baseless accusations, they now openly challenge her for the throne.
Enter our heroes, 5 individuals of recent achievement and recently recognized for their achievements in Gaverdon. They have now been contacted by their queen and asked to do something no one else has yet to achieve. Tasked with retrieving a mysterious artifact from the ruins of the old keep, they must journey deep into the heart of the old city to achieve their goal. However, the mystery remains of why these nobles can so boldly rebel against their ruler and rally the support of their people. What do they know that the players don't? What is this artifact going to do? Has the queen been completely honest with the players? This is what I intend to wrap up in the second part of this adventure.
The Setting
The players approach the ruins of the old castle that used to house the royal family as well as many members of the court of Gaverdon. The place is now decrepit, and warped by magical energies as well as the taint left by the devils as they were forced back into the nine hells. It is now haunted by a host of spirits and undead of both good and evil, from crazed spirits who lust after their former lives, to eternal guardians who will not rest until their task is complete and the mystery within is put to rest.
The Plot
The current queen of Gaverdon is a legitimate heir, in that she is the offspring of the former king. It is not so widely known however, that she was born to a common woman and not the former queen. Though aware, the queen kept quiet in order to preserve the unity of the kingdom during dark times. The people needed to know that there was an heir to the throne and claimed the child as her own. As for her mother, she was given a job at the castle so that she could remain close to her child on condition that she never let it be known that the child was illegitimate.
Life continued happily around the castle for many years, with the king mostly away on campaign against the wild goblin tribes that inhabit the south of Gaverdon's political borders or the orc clans to the east. Upon return from a recent campaign, the king spent the night with his wife after being away from home for almost a year. During that time an emergency struck that required him to return to the front and he departed within the week. It was discovered shortly after, the queen was pregnant. To many this wouldn't come as a surprise, but the queen had been declared unable to have children early in her life, and now she was having a child. But what of Estel? This peasant born girl was now the recognized heir to the kingdom, something not easily retracted without raising questions that could not be answered.
The king was gone for 11 months, enough time for the queen to have her child without him ever knowing about it. She then made the most painful decision of her life. She would keep knowledge of the child from him in order to preserve their marriage and the security of their kingdom. She ordered a secret room built in the castle to house the child and swore all the servants to secrecy on pain of death. From there she would continue to care for this child in absolute secrecy, and when the city was destroyed she died with him, unable to leave her own flesh and blood whilst the king took what he believed to be their real daughter with him to safety. Both the queen and Estel's true mother would not leave the city alive when the Devils came.
The artifact the PCs seek is in that very same room that contained the queens child, and locating it will be the challenge set before them. They must search the keep and learn what truly happened in the royal house of Gaverdon. They will meet spirits both helpful and harmful, including the benevolent spirit of Estels mother and the twisted apparition of the former queen, fraught with rage at the denial of her bloodline. Ghosts are not all that inhabit the castle, and they will have to
deal with abominations twisted by the hellish energy that permeates the castle and ghastly corporeal undead that have risen to stalk the halls and prevent would be treasure hunters from solving the mystery and locating the artifact.
sounds wonderful! Not to nitpick (though you DID ask for it) - you mention "abominations", as this is hellish and not far realm, is it safe to assume you will NOT be using aberrant creatures?
It seems rather straight forward. By that I mean to the players its just going to look like another smash and grab.
I don't think there is anything to make them suspicious of the queen's motives, she may be lacking in ability but she is the rightful heir and better yet she is leaning on them for this. A good patron is awesome for the PC's, so unless you feed them information to make them suspicious they should do what has been asked without issues.
Now if they should succeed and return with this artifact and be betrayed in some way by the queen, bad bad things will happen (well they would in my games where the PC's care about their image and reputations as much as their hitpoints). This may be exactly the plot you are looking for but its hard to tell where you intend to go with it.
I would urge to get the players more involved in the plot, they could be approached by an opponent to the queen who will urge to them that she is misleading them...The artifact is dangerous and will bring the horror that was placed upon the old kingdom down on their heads again. Or that there is no artifact, the queen is trying to lead the heroes to their doom because she fears they will be tools used to overthrow her.
This is also a good opportunity to dispense different elements of the plot to different players in isolation and only if they all bring what they know to the table and share it will they be able to possibly piece together the puzzle. Take them outside one at a time and give them their bits of info, don't rely on them playing in character should you tell them everything at the table. This creates a fun mystery type of atmosphere at the table, especially if some of the players think sharing their info will do more harm than good.
I think you've hit some very good points Tom. I think there is potential to think about. I've had a long day at the office so I'm not quite in thinking mode but let me lay down an idea that may have potential. I'm thinking that since it has only been a short time, perhaps the true heir has "survived" in that it's possible that he's been living as an undead (a custom undead creature of some sort... one that isn't of the inherently mindless and evil variety).
I also like your idea about approaching the players before the session, and giving them information that their characters will have access to and no one else. I like the idea of things not being common knowledge amongst the party. It is very possible for some of them to have doubts and misgivings about their task should they learn that they have been decieved even if unwittingly. The groups paladin, for example, is an older man who lived and served during the time of the now deceased king and is fiercely loyal to his family (hence why he's on this adventure) while our artificer has expressed his doubts from the start already saying things like "an artifact that can save a kingdom, prepostrous! There is something more to this."
So, I'm interested to see how the paladin will react to learning about this scandal. Especially if the heir is "alive" he has a choice... support a leader who his people disrespect and maintain a state of order or challenge her right to the throne as well and champion the surviving heir.
Of course, that's just one of many paths I could take this... feel free to drop more ideas I like the way you think. 
EDIT: Of course, feel free to tell me I'm overthinking this to much and should just roll with something fairly straightforward... I don't mind it as it is I just think it could do more...
Thanks, I am glad you like my devious approach to things. Hehe.
I don't think it ever hurts to overthink things, you should be several steps ahead of your players and you should be equipped with a knowledge of what makes the relavent NPC's tick.
If you can accurately second guess how your players will react to the situations you set before them then you are really ahead of the game, but in my experience you just can be sure until it all unfolds...at which point your best ability to roll and flow with the outcomes is what will make an entertaining game.
Bottom line for me is I want the players to have fun, get emotionally attached to their character and the world they are in and become emersed in the experience.
Solo runs before the main campaign (take them to level 2 or 3 in solo work) could establish a nice variety of points of view. Each could be a recent hero to the kingdom - but to what end and in who's eyes? The heroes could have each worked for a different noble house, some friendly to the queen, some neutral, some opposed.
Friendly heroes might have a hard time w/ wierd layers especially if you establish 'facts' during the solo work. (Maybe one of them saved a seer that pointed them in this direction)
Neutral heroes might be more cerebral, having already been made privy to the dilemma and being interested in finding out more truth.
Opposed heroes might be sent to find 'dirt' on the queen.