Well, Sean, it sounds like you want a quest similar to something Red as well as I have made permutations of. His idea is that a Giant Demon on a Volcanoe is created and demonic armies spew forth until someone kills the demon.
*My* idea, on the other hand, is a chain of events that starts with three rather Innocuous Bounty Hunter quests and snow-balls into a VERY bountiful quest that has the potential to rend the fabric of the world as we know it. Basically one of the possible outcomes is to inadvertently spawn a completely new (playable?) race, based soley on demons and monsters, with some minimal involvement of Fallen. They even have their own Channeler and everything.
za?? you ask? Well, if your feeling kind of arrogant or Goody-Goody, you can *Attempt* to kill the Channeler before his Nation of Demons and Monsters has a chance to get off the ground. Also, you can get a pretty DAMN nice sword for all your troubles, not to mention all the baddies you get to kill, cool cutscene potential, and interesting gameplay + interesting decisions. Potential Diplomatic consequences abound. Travelling the earth with an adventuring party/army also hold significant influence. You might have to travel the four corners of the Planet in an attempt to complete the Opening stages.
The beggining involves having three loosely connected quests to kill a Giant Monster of some-sort. The Cannibalisitic Troll of Death-Maw valley, the Hellbeast of the Three-Tree Grove, and the slaying of the Mighty Manticore of abandoned city Alusberry (as a rough example). Perhaps in the initial request you are supposed to gather a piece of the creature. However, each once you kill each creature (and collect the sample) their body is eaten by the earth. (optional, it could turn into a Spawning ground for monsters).
Once all three creatures are defeated ... the earth opens up somewhere in the map, and from the hole a Darkened Castle rises to fill the void. The skies around the castle are ruled by Gargoyles and the Occasional Edar Lord(or prince).The Castle Halls are armed to the teeth with elite Demon Royal Guard. The treasure Vault is guarded by a small caste of Fire Sions and Air Sions. There is a central Royal Champer from where the (insert big bad guy here)* resides.
*Im thinking A great DemiDemon-lord. (Large Unique Champion of Demonic descent - looks some-what more human than the "demons" of the Creature Catalogue, but still very reptilian/alien-much moreso than the Fallen).
Past the Royal Chamber is a stairwell that climbs deep down into the depths of the Earth. Within is a great maze of Caverns and traps, riddled with demons and other magically altered beasts. At the center of the Cavernous Dungeon within the Earth, theirin lies the TRUE big bad. (really REALLY bad guy-either DemonLord or Titan's Memory, or some other such Super-sentient creature with command over Fallen Creatures and beasts of the Earth).
For the most part, the Castle remains still, and other than the occasional raid by gargoyles led by an Edar with no fealties, the residents remain content within their Castle Domain. The Door is locked, although the Castle Wall has a large ladder one can use to climb to the roof of the castle, where from there one can battle at least the Gargoyle and Edar residents atop the Lengthy series of Battlements. And one might possibly (with a high enough survivalist or detect skill/spell) find a hidden door into the interior of the Castle.
Meanwhile, The person with the Three Creatures slain will attain a map. (either the pieces collected by the fallen creatures will transform into a map, or you will be visted in the night *triggered event* by a shade-spirit).
The Map will point out the Location of three treasures (pieces of a sword) which are hidden within distinct geographical locatoins. Im thinking one high up in the mountains/caverns/tundra, one in the depths of a great Swamp- perhaps guarded by a SwampBeast, and a final one in the harshest heat of a scorching desert (scorpions, wandering scions, guarded by a Sphyinxish character??). The map hints that these pieces can be forged into a mighty sword. Once all three pieces are collected the map reveals the location of a Hermit, fore only he can Forge the sword within the mighty Fires of X (im thinking magically built BlacksmithShop/Cavern within a natural dormant-ish Volcanoe). If you forge the weapon, you realise that it was made by its original master as a Blade of Vengeance, and that the dying wish of its creator is that whoever wielded it would come to kill (Big Bad, although perhaps not specifically referred to). The map will mention that the Creator constructed the map, and had hidden the blade pieces before he died. The map referrs to the "Lord of the Castle" and that once your blade is drenched in the blood of the Lord, the blade will transform into a weapon of untold arcane power.
As it is, the Blade of Vengeance is fabulously powerful against Sentient Humanoid creatures. 50% damage against human and fallen, and 100% damage against demon-kin. indeed, a very fast blade, and it seems to drain strength from its enemies while you kill them (either increased stamina, or attack speed, or hit-points, something useful yet temporary, yet not too temporary to wear off before the battle is done, probably increased morale as well, while lowering morale of opponents, drinking of the blood spilt). *-if the Blade of Vengeance is soaked in dragon's blood, its wielder gains a single power of dragons, as well as the ability of magical flight.
Using the blade, you can enter the castle (or maybe forging of the blade causes the Castle Door to open fully). Either way, once you approach the castle, its denizens are awoken, and you might have to fight off an attacking band of Demon-Honor Guard led by an Edar prince or two (before you even enter the castle). In fact, as long as the Castle Door is open, there is always a change for an exploratory demonic force to expand from the castle. (therefore, if someone lives near the Castle, and its not the quester, it might be in their best interests to stand guard in front of the castle, in an attempt to keep the quester from entering (without fighting/killing you first). Then they would have to decide if fighting you is worth entering the castle, ect. Although ... if merely *Forging* the blade will open the castle doors, then the Local nation would actually perhaps *encourage* the quester to enter the castle.
This brings me to my next idea, (although im still not finished with the quest). The Sovereign and Party don't have to do the entire dungeon all in one *instance* or one turn. Instead they can travel along it as an Alt-Map, and when enemies approach they enter a small battle-screen (much like an RPG? eh? eh?), of course the difference is that the enemies aren't randomly generated, but already in the castle (although their location and number within the castle could initially be randomly generated). Also, if Living Lairs are implemented, the Castle with an open door should, in my opinion, become a living lair. So as the Channeler and his party is questing through the castle and under-depths, he would be able to go a certain distance/fight a certain amount of battles, and then wait in the dungeon for the next turn. (dungeon crawling could take more than one map-turn) - and this dungeon especially. However, if you accidentally stumble into the Castle's Lunch-hall, or the barracks, then youll be faced with a significantly "large" battle ... hehehe.
As with living lairs, ect, if the party does not enter the castle soon, or if nothing is done for some time (or the quester dies "gasp!" or the blade is somehow lost) ... then its possible for the Demi-Demonlord (Midboss) to ride out of the castle on his prized epic/elder Umberdroth. Travelling with him would be a massive twisted army, that half exits out the castle and half rises from the ground itself. The mass-conent of the army would be lowly creatures (perhaps hordes of skeleton/undead), some ghastly horrors, some scions, a Core Fighting Force of elite Demon-honor guards, and aerial support from a squad of gargoyles and a couple Edars. In this case the rest of the Gargoyles would remain to protect the castle, and the Door to the Castle would close (the Demi-Demon has the key with him, and only if you defeat him can you enter the Castle).
Once you kill the Demi-Demon (or someone else kills him), the map reveals an underground chamber beneath the Castle, where the TRUE big-bad resides. The map goes into more detail of the instruction of who the real Big Bad is, how to kill him, roughly where he is, and an idea of what is awaiting you in the Underhalls. With the exception of the demon-honor guard, the Vault-guarding sions, or the Grande Army of the Demi-Lord invaded the realm of living with his troops ... this is where things begin to get tough.
Traps could do things like *poison* some units, *blind* some units (temporary effects), or any number of ill effects (as long as there are buyable potions somewhere topside, and/or magical countermeasures) ... actually I have an idea that along the Adventurer techpath you can research "magic shops" ... and you can create a magic shop where champions can buy semi-rare artifacts, as well as Potions for a wide range of prices, with artifacts being worth a small fortune and potions a mere month's pay.
Traps could also shift the earth of the caverns, changing how the Caverns interconnect ... making a new maze (this one's my favorite). There will be treasure rooms guarded by deadly beasts, as well as large chasms filled with the undead or other unsightly horrors of the Under-Earth.
Once you fight the TRUE big bad, it will probably be the largest cavern (AKA the largest/biggest battle, with perhaps an exception of the Castle Barracks). A swarm of edars overhead, as well as a field of zombies and skeletons to fight through. The Big bad would probably have armor made from the bodies of fallen foe, perhaps using Skullbones to form a helmet, and other souveneirs along his armor. He would be very unique, perhaps tied in with the Lore. In any event, once you slay him, if any of his minions in-combat are still alive they will withdraw from the battlefield, once more into the shadows. Then, soaked in his blood, the Blade of Vengeance becomes the Blade of Supremacy. The sword itself becomes slightly stronger, yet the bonus vs demons dissapears. It gains an Attk/Def increase, as well as a HitPoint bonus to the wielder. It will also regain one hitpoint per kill to the wielder, as well as compile bonus hitpoints atop his max for kills beyond max health. These are temporary hitpoints that would dissapear after the battle. The Sword of Supremacy would have a 100% bonus vs Humans and a 100% bonus towards fallen, but no bonus towards any demon-kin or other Fallen or Living creature.
The Blade of Supremacy's main strength, however, would be its Arcane Powers. Firstly, it gains a +1 Arcane attack (if thats possible). Also, for every 50 kills it can unleash a fearsome arcane blast (spell) which shoots from the blade in a cone much like Dragon's Breath. Hopefully its possible to store 50 kills to bring a blast from one battle to another battle some turns down the road even.
Another thing that happens once the Big Bad is slain, is that a Gigantic Statue (in the Castle's main Throneroom) that was once admired by human, fallen, and demon alike, comes alive. He sends an emmisary to you, who happens to be a demon within the room (or even a shade-spirit like at the beginning of the Quest). He thanks you for giving his Castle back to him from these Usurpers, and that you will be rewarded well. The blade is his gift to you, as long as you sign a 20 turn Non-Aggression pact, as well as strongly consider his offer for alliance (accepting alliance will grant increased benefit/rewards, as well as having diplomatic consequences). If however, you refuse his offer, you will have to fight your way back into the Castle and kill him, cause you can't escape the Castle unless he dies, cause he would Seal the Door and other possible exits. If you choose to fight him, it would be in the Grande Throneroom, where you first saw the impressive Statue. He can (probably) transform down into a human-size and form if he wants to, but his Demonic descent is very clear (or Titan, who knows/cares .... he is a Channeler/Magic user as powerful as any Soveriegn, and thats all we need to know
). To repeat, while the first Big Bad was a Champion, and the second Big Bad was even stronger (Titan, DemonLord, ect) ... the Statue is a SOVEREIGN!!! A Sovereign of Monsters and with full Channeler abilities and abilities as a Ruler too. If you fight him it will be essentially like fighting an enemy Sovereign in their own home-lair. Not a pretty fight. Im not sure if death would actually result in Death=gameOver or simple imprisonment. Im inclined to think that you "respawn" in a Dungeon room of the Lower Caverns as a Prisoner. If thats the case, since its *unfun* to simply sit in a Dungeon and wait to be rescued, perhaps you would have no access to your Mana (until you are rescued) and can infinitely try to fight your way out without your Magical abilities (sealed by the Demon-Sovereign, or whatever he is). On the OTHER side of the coin, in the real world, your empire could still be controlled by you (optional), but at significant penalty of course. For one, you wouldn't have a Sovereign or any of his Global Spell abilities, ect. Perhaps that is Handicap enough. In any event, you can try to Diplomatically pursuade your friends to rescue you, or lead an operation by yourself against a MAGICALLY POWERED sovereign fight on enemy turf.
Regardless of whether you try to kill this New Bad or not, if he remains alive (your friends or a prisoner) he becomes a new leader to interact with globally. He gains some diplomatic bonuses with the Empire, and some Diplomatic malus's with the Kingdoms. For the most part his units are all Creatures used by the Fallen and Undead, as well as any other Magical or Mundane creatures that could be ruled by the ruler of such a Dark Castle of the under-earth. This would include Edars, Umberdroths, Demons, and Skeleton Warriors. He could also aquire trolls, or even a Dragon, but only through normal routes that are available to all other nations.
The Castle becomes the Keep of his first City .... his Capital of Beasts. His population is not fallen or human, but demon. As such they are far fewer in number, and yet far more effective (to remain competitive). He can equip demons with weapons and armor, or he can recruit monsters and undead. Monsters and Undead are not bartered with or summoned with spells (well, maybe with the Undead), but are instead constructed by Magical Resources, much like the Social Races would construct seige engines. The building of monsters is much like the recruitment of a unit, however. He picks which monster, and decides how long to train them, and in what sized grouping. Dark Castles are the only Keeps that can build Magical Creatures in such fashion, and only by this Sovereign/Leader and his faction *special*. Im not sure if these demons should eat food or not, but they should be the population of all of his cities. Thus, in the case of attacking *his* cities, its a bit more of a daunting prospect. Also, im imagining that his cities are also populated by Undead Slaves, however they are probably not counted as Citizens (for the purposes of wealth and commerce). He can build buildings normally ... possibly having his own set of buildings and techs? At least buildings perhaps, and could steal the tech tree of some Fallen Faction. Probably Each "Citizen" could count as 10 citizens of another race (maybe even 50-100?). In any case, housing would have to be completely different, as well as functions of prestige (since how can humans migrate in, or demons migrate out?).... population growth would have to work in a dynamically different way. Dark Castles can only be built upon Shards/Mana nodes. All other cities function as normal Fallen cities, with fallen buildings and fallen population. This Royal can breed with Fallen or Human, however all children are either demon or fallen. Non-Dark Castle cities have fallen population, acting *normally* however Dark Castle cities use Demonic population, special buildings *perhaps*, and can build magical beasts.
In every way this new Sovereign becomes another AI leader, with his own agendas of World Domination, ect. Perhaps the only path he cannot achieve victory is the Questing path *maybe*. However mastery over Magic or Military Might are possibilities for him. Especially due to his relative ease of aquiring Edars, Demons, and Undead (among others). He starts out with a relatively massive treasury, and a decent army, and a highly productive Keep with many magical resources. Perhaps the higher "grade" of the Shard, the faster a new Dark Keep can build Monsters. (still an option for all cities to have demonic population, would have to be a Dev decision).
Another dev (or modder) decision would have to be wether to allow the Dark Keeps to simply be "demonic" population, or an assumed mix of demons and fallen. In the latter case, all cities could act normally (except the building magical creatures part) and all cities could build soldiers of fallen. The catch, of course, is that All cities *also* have demons, and so an equivalent unit can be created with demon troops (stronger and more expensive).
If possible, all Royal Lineage should be half-demon (Demi-demon). Or at least Fallen of some sort, or some other type of monster. If you kill this Sovereign, all Dark Castles become NPC living lairs/dungeons, and all "normal" cities become Independent (with the added bonus of access to demon troops). Also, the Sovereign changes back into a Statue. This statue can be placed in the City square of any of your settlements for a *HUGE* Prestige boost. Statue can be moved from city to city, although full Prestige value of the Statue takes 10 turns to accumulate (snowballs into a larger per-turn bonus), so its best to leave it in one city.
If you destroy a Dark Castle in some way ... pick it apart/burn it to the ground, ect, your Channeler can use his Revive Earth spell on the ruins, or some other Channeler only skill which may or may not cost essence. In any case, once you use your spell on the ruins, the CastlePieces are lifted away, and once again you have your Mana Shard free to tap into.
As long as the Demon-Channeler AI Sovereign is alive however, the player that freed him gets significant advantages in some way, and the frequency of Barbarian Magical Creatures increases.