Backstory:
When Io was defeated by the elementals, the way it was done was actually quite sneaky. The elementals worked to pull out the aspect of Io that kept His two sides in balance(Bahamut and Tiamat), thus causing him to destroy himself in the battle for control. This is how Bahamut and Tiamat came into existence, and they turned on the elementals. In the ensuing combat, that piece of divinity robbed from Io was forgotten.
That is, until Corellon discovered it, and She/He/It decided to put it to good use. Having no need for internal balance like Io had, she crafted it into an artifact, the Golden Scales. This artifact, depending on the user, could both measure, and control peoples intentions and motivations. It could, for example, wielded by a powerful enough God, turn an Evil God to Neutral for an amount of time.
Realizing the power of what she had made, but unwilling to destroy it, she decided to protect it. With the assistance of the other unaligned Gods, She crafted a pocket dimension, extant from all other dimensions, that comprises a large and ringed maze. Several hundred kilometers wide, it cannot be entered by either Divine, Devilish, or Demonic beings. Only mortals can enter.
She placed traps, puzzles, and guardians throughout the expansive maze, to defend against the mortals influenced by either Divine or Demonic reasons. Mostly the maze consists of concentric rings, filled with varying kinds of terrain, mountains, farmland, even a desert.
The maze portion comes from the fact that to go into the smaller circle of the one you are at, you have to first find the entrance, brave the entrance and its challenges, and then survive whats on the other side.
Time as well, flows slower in the Maze as it does outside. Every four months outside, the maze appears for a week, in a random place of the world, one entrance open to all. Corellon designed it that way, so that the mortals could attempt to challenge the maze, and in so doing, admire what she had wrought. She considered it a success with every death that racked up.
The maze has been doing this for thousands of years, with whole expeditions venturing within... and never coming back out. The previous human Empire had lost an entire army and one of their greatest generals to the maze.
Within the maze, in the large tracts of available land, people have settled. Despite the dangers of the maze, with constant attacks, traps, seemingly random events happening almost weekly, they thrived. The hardier and braver races and groups ventured further within, becoming more and more skilled with progress.
However, as each ring has less land, the pressures between groups becomes more and more severe, leaving the last and smallest ring wracked with seemingly eternal warfare.
Outside the ring, there are forces positioning their pieces for a war for the Material Plane. One of the key players is Ker'u Bael, a greater demon lord that has been trapped in the Material Plane in insubstantial form for an epoch, thanks to the efforts of a particularly powerful cleric. Unfortunately, Ker'u Bael had the last laugh, finally corrupting the nameless Cleric just before he achieved Demi-godhood, and thus condemning the Cleric to an eternal hell.
Luckily, the effects of the nameless Cleric's efforts kept Ker'u Bael trapped on the material plane, and none in the Nine Hells particularly cared, since it left an entire realm up for grabs. Ker'u Bael is determined to turn the Material Plane into a new Realm of Hell, and thus gain back his prestige, power, and minions. For that, he needs the Golden Scales. Bael can only manifest as a simple shade, only able to whisper and corrupt people. He controls people effortlessly, thanks to long centuries of practice.
In opposition to Bael's plans, are a large group of forces, both demonic and divine. Bahamut has a particular interest, because it was his Chosen Cleric that Bael had corrupted. The demon lord(nameless, because his full name cannot be pronounced without going insane and becoming his vassal) that took Bael's place has a vested interest as well, since he wants to keep Bael in his place. Note that Bahamut and the nameless one are not aligned, just have a similar interest in stopping Bael.
In addition, the unaligned Gods fear a war between the Good and the Evil forces, and want to prevent any such catastrophe from happening.
Now, what this means is that the PCs will have various motivations, and they band together at the entrance to the Maze. They could have been instructed by Bael himself, of through a puppet, by one of the Gods with a stake, or by Bael's demonic opposition.
I also considered, once the PCs are aware of Bael, and his plans, having a bit of a timewarp, for fun, and have them at the Final Battle with Bael's forces, all of them at level 30, fully decked out in magical armour, weapons, the whole bunch. In addition, they'd have a Holy Army behind them. Personally, that would be absolutely kickass, and give the players a bit of a hint of what will happen when they finally get out of the Maze.
There are betrayals ahead for them, maybe even from their own party, questioning their own motives, who they work for, and just why they do what they do. Hence, the name, Balance of the Heart. The idea is to play with the high fantasy stuff, epic war, fate of the world, and still be dark, moody and dangerous.
Thats pretty much what I've got so far. Any tips, critique, questions?
The proposed ending seems good as well but one word of caution; anytime I've been in a campaign where the DM wanted a certain outcome there was MASSIVE railroading. Railroading sucks.
Other than that, have fun.
As stated by Voidraithe allow the PCs to develop their characters some before they jump into the overall campaign. This doesn't mean that the first couple of adventures can't have anything to do with the overall campaign, it just means that they should be ignorant of the plot until later on.
As also mentioned by Voidraithe, try not to railroad the PCs. Instead give them several options, hooks, that seem to head off into different directions but eventually converge at the same point. This gives the players freedom of choice but also keeps them involved in your campaign.
Give new players a chance to play with high level characters, and to... give them a taste of what could happen in the future. In that session, I will be trying very very hard to kill their characters, and being epic level makes it very dramatic. Its kind of a... "This is a possible future you guys may want to avoid... just as a hint."
Ker'u Bael is a Greater Demon Lord, once master of a level of Hell. But having been summoned, and trapped on the material plane as an insubstantial wisp, kind of ****** him off. He only has power when someone lets him into their bodies willingly, and even that is a shadow of his former might.
How do you mean a more vested interest Voidraithe? For the players to get the Golden Scales?
However this means you have to develop friendships with NPC's and have them based out a town which is why I suggested taking it slow at the begining and developing those motivations. Bieng manipulated by Devils or sent on a quest by a god are good but nothing motivates PC's like a personal intrest.
Thats what the future vision is for, to show the players dramatically why they need to stop him as soon as possible, or half the world will be covered in death and destruction.