Dragon Magazine once published a miniseries called Libram X, with a story by Jeff Grubb (and later Dave Gross) and artwork by Bob and Cathy Lessl. It came out just when they started advertising for Planescape, and I assumed that this was what Planescape was going to be. It turned out to be something different, but I thought it'd be interesting to combine the two. One interesting thing about the series, which I don't mention below, is that the characters will often meet alternate versions of themselves. There's no reason this couldn't happen in Sigil as well.

If you haven't read the comic yet (in Dragon Magazine, the early 200s), be warned that there are spoilers below.

The Mazeworks, where the war between Law and Chaos rages on in ignorance of the city above.

The Mazeworks are technically part of Sigil, but a body'd never know that from looking at them. They're far beneath the city's streets, and all that can be seen are endless tunnels, staircases, and rooms, clearly of artificial make but with no apparent limit. The halls of the Mazeworks often defy the logic of space, gravity, and perspective as they double back on one another, suddenly shift 'downs,' and overlap in confusing ways. As in the rest of Sigil, every doorway, window, trapdoor and arch is a potential gate to another plane of existence for those who know the keys, and while certain landmarks remain, the layout changes over time. There are no legends of the Lady of Pain here, only allusions to a mythical Architect who left behind 'his' book -- the Libram X -- in some unknown part of the 'Works. The Libram X is said to have the power to remake any aspect of the Mazeworks -- the locations of portals, the structure and contents of rooms and passageways, and the very people inside. As a result, every faction wants to get a hold of it.

Most of the people of the Mazeworks think of their home as the "real" world, and all other planes mere reflections of it. Although this attitude is found in upper Sigil as well, it is much more common here.

The major power groups are the Lawboys -- a motley collection of kytons, baatezu, mind flayers, and undead bent on taking control of the 'Works and bending it to their idea of order -- and the forces of Chaos, who, much like Sigil's Free League, only want to be left to their own devices, but who are willing to fight the Lawboys in order to make that happen with a fury worthy of the githzerai or bariaur -- two races that are occasionally seen among them. The populace of the Mazeworks is, like Sigil's, incredibly varied, full of beast-headed or headless humanoids, sentient zombies (possibly from the Silent Kingdom), humans, gnomes, tieflings, genasi, tanar'ri, gargantuan talking mice, centaurs, and more. For some reason, hordes of mites and jermalaines are often found trailing the Lawboys, looting their victims and cravenly obeying their wishes. The technology level is slightly higher than in the Sigil most know, with pistols and cannons fairly common weapons, and even the occasional personal computer magically altered to talk to the Mazeworks' thin network of web servers without electricity.

The secret no one knows is that the Libram X has been found a number of times before. It's usually hidden within complicated illusions, but it can and has been discovered by those who know the proper darks. The book itself is ordinary, except for the weak enchantment that prevents it from being written in by anything but its accompanying pen. The pen is the real artifact -- it's the one known to sages as Kuroth's Quill.

Origin

The true history of the Mazeworks goes like this: at one time in Sigil's distant past a stranger came into the Cage bearing the quill of Kuroth. He attempted to use it to take control of the city for himself, writing "I control this city, and not the Lady" in his journal.

Almost instantly, he found himself mazed by the Lady of Pain. Like her, he was the absolute ruler of the domain he found himself in, but he had no way out.

The stranger wasn't done yet, though; for whatever reason, he still had his Quill. In his journal he wrote, "I am the master of this Maze, but this maze is Sigil. It exists in Sigil's place, and functions like Sigil in every way. The Lady of Pain and her dabus are nowhere to be found, and cannot interfere with this place in any way."

His wish was granted. The Maze he had been trapped in moved into Ligis, Sigil's Undercity (arguably, all Mazes are part of Ligis, but that's literally neither here nor there). Portals just like Sigil's were activated throughout it, and the Lady of Pain and her dabus were nowhere to be found.

The stranger, however, vanished. Most likely he was flayed alive, his remains used for fertilizer in the dabus warrens. It seems that at the time he was changing the nature of his Maze, he had become no longer a part of it, and in that time the Lady was free to act on him without violating the wording of his wish.

The quill, however, was left in the Mazeworks, hidden by some unforeseen side effect of the wish. To this day, whenever someone finds it, they are no longer a part of the 'Works, and if they don't quickly try to use the Quill to bring back the Lady of Pain they end up mazed, and the Quill vanishes to be hidden somewhere else.