Alright, this isn't so much about mechanical questions as it is about seeking help fleshing out an adventure I'm writing. I have a rather small amount of experience playerside and zero DMing (But have had large amounts of exposure to lore and game concepts through PC games, source books, etc), so at the moment I am focusing on the writing aspect and waiting to map out the nitty gritty details until I'm nice and comfortable with the rules and such. This thread's here to seek advice, suggestions, or feedback from DMs with some experience in the story telling and lore aspects of the game.
To start with, I'm using a rather tweaked incarnation of the Forgotten Realms setting. Basically just moving forward in the timeline and overwriting the, in my opinion, apocryphal story events and cosmology changes implemented in 4e FR. The pivotal concept of the change being an event called the “Portal Storm”, which caused an abnormally large amount of portals to open and close all over Toril over the course of about a decade or so. This even caused a large amount of planar creatures both sentient and otherwise to be stranded in Faerûn and beyond, and enough time has passed since this event (perhaps a couple centuries or so) to allow these planars on Toril to begin mixing and integrating into the environments and societies of the natives. These beings mostly consisting of aasimars, tieflings, gith, and especially genasi (I've got a thing for them), along with other planars exotic and otherwise that would have the ability to integrate at least somewhat successfully with existing Faerûnian cultures or start their own (I would greatly appreciate suggestions). Along with other changes such as more solidified nations and fewer independent city-states like those on the Sword Coast, and other things. The more wide reaching aspects of the setting are not going to be explored too much in this adventure, but will be in the larger campaign that the adventure will sort of begin.
Now! On to the adventure itself! I haven't ironed out many details about the beginning of the adventure. What I've worked out so far is that the party is hired on by a dust genasi archeologist as part of an expedition to a strange archeological wonder. This wonder being a sort of floating island, hovering and perhaps drifting many hundreds of feet or more in the air. Built on and within this island is a large ruined citadel of a build unfamiliar to most who would see it, and clearly very ancient. How the party reaches the island I haven't figured out, but after they reach the citadel they would begin the action with perhaps some sort of strange (low level) creatures living on the surface of the island, up until this point I have few details solidly worked out. Once the expedition enters the citadel however, I have a better plan for the story. After a bit of exploring and critter slaying in the top levels of the citadel, the expedition would be suddenly attacked by a huge behemoth of a creature. It is made as clear as possible that this monstrosity it well beyond what the party can handle, and they must flee. In the following chase scene, there is eventually a large collapse caused by the beast's smashing about the corridors. The party (which up until this point has consisted of many people other then the players) is sent smashing through an unknown number of floors into the depths of the ancient citadel, and when the dust settles and people start digging themselves from the rubble, it is quickly learned that the players are the only ones who have survived, leaving them stranded on an unknown level deep within the citadel. This establishes the overall goal of the adventure, which is simply: Escape. The party needs to find their way out of the citadel, somehow avoiding the behemoth on the upper levels and finding a way to return safely to the ground below.
This is when things really start to get interesting. As the party gets their bearings and begins to wander the citadel, they eventually encounter (the details of the encounter scene I haven't figured out) sentient denizens living in the ruins. These denizens consist of two opposing groups, one being a society of sentient constructs left behind by a powerful wizard who seemed to have used an area of the ruins as his lair, the other a society of sentient animated skeletons. The constructs were granted sentient thought for mysterious reasons lost to the construct's knowledge though arcane devices of a very exotic nature, and the skeletons were originally mindless undead guardians left behind by the citadel's initial inhabitants who were bestowed with intelligence through power leaked from one the wizard's abandoned creations that was drawn and gathered by magical gemstones placed on each skeleton's forehead. These two factions have been warring for many, many years, fighting over one of the old wizards largest artifacts, the details of I haven't worked out much of, other then it is a large orb of some kind. The orb's purpose seems to be to keep the constructs bound to the citadel, causing great pain should they ever leave to the surface of the floating island and quickly killing them should they ever try and leave, however, this artifact's leaked power is what allowed the skeletons to develop their sentience, making it the centerpiece of a war the two groups fight. The constructs seek to destroy the orb in order to end its grip on their minds and allowing them to escape the citadel, while the skeletons fight endlessly to protect the orb, and with it their continued existence. These sides have been for the most part evenly matched, their war largely at a stalemate, that is until the party is introduced to tip the balance of power. I want to make it very clear to the players that these groups are not the usual good vs. evil sort of pairing one might expect, if anything, it is more a conflict of chaotic against lawful (The constructs being chaotic and the undead lawful).
Wow... that's a lot more writing then I thought I was going to do, and it's rather late, so I will finish this up another time (There's still plenty of stuff and story to tell). Feel free to give feedback or suggestions or whatever in the meantime.
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A) For the Portal Storm, I'd ask what creatures in Faerun were forewarned about events to come. Haunted Halls of Eveningstar and Magic of Faerun have important sidebars on mages obsessed with eschatology.
I'd also ask what pressures have lessened along the Sword Coast to allow for more discrete nations. If you're familiar with the maps, the city-states of the Western Heartlands largely exist as points of human civilization broken up by large horrors or menaces - a monarchial Waterdeep, for example, would have to deal with issues in the Mere of Dead Men or the Kryptgarden Forest. If you have access to either a copy of the Forgotten Realms Campaign Guide for 4e or Steven Schend's Blackstaff, look at the High Moor therein for a possible major city-state that wouldn't be on maps in the 1370s. I'd also look at Lost Empires of Faerun, Serpent Kingdoms, and the later works on the borders of the Silver Marches and Cormyr for possible ideas of what might create borders for your states.
C) A perfect place for your floating island to have come from is a chunk of ruin freed from the Boneyard (Underdark 130.) Certainly a bit exotic, but it's also close enough to old Raumathari ruins for the constructs to make sense (see their write up in Lost Empires for why I'm specifically suggesting them.)