How to destroy Sigil

In my opinion -- a good story > ANY game mechanic.

The Lady of Pain (in my view) is there for three reasons (of the out of character kind):
1) to keep Sigil relatively stable as a setting for PCs to adventure in
2) to allow races that would otherwise slaughter each other (fiends/celestials for example) to exist in the same space for roleplaying reasons;
3) to remind player characters that there is ALWAYS something bigger than them that can easily give them and their stacks of +5 swords a beatdown.

As a DM, if you want the Lady of Pain to be overthrown, do it!

Yes, of course there would be cosmic consequences. First off, Sigil is literally located in the center of the multiverse -- it's a great big floating donut at the center of the Spire, in the middle of the Outlands. I do find myself thinking that if something upset the city, especially in a major alignment-related sense, it could have consequences for all of the Outer Planes. I wonder if the physical stability or survival of the Planes would be threatened, in a structural way.

I think it might be better for the Lady to be displaced/exiled/disappeared than clearly killed off; you'd always have the option to return Her to power later if you wished to.

As for how...

The Lady's more powerful than a deity (she can bar greater Powers from her city with ease). I find myself thinking that something about the nature of Sigil has its own set of metaphysical laws, unique anywhere in the multiverse -- and so far, only the Lady of Pain knows the secret. Perhaps in your campaign, someone else has figured out the dark of it and learned how to exploit it, without getting mazed or flayed in the process. (Or perhaps the person WAS mazed but found out how to escape...)

My own train of thought is leading me to an evil, misguided or simply renegade Guvner. Perhaps he went with the theory that all planes have their own metaphysical laws, and that Sigil acts as a unique plane/demiplane with its own set of laws. The one who fully understands the 'physics' of Sigil, wins. Unfortunately, either the baatezu got ahold of him, or (if he was LE) he gave that knowledge to them himself.

Now, normally, I'd never recommend that the Lady of Pain be overthrown -- Her existence is one of the cornerstones of the Planescape setting. But if you want to in your campaign, do it.

According to Planes of Law, the baatezu in Dis are building an artificial replica of Sigil in order to study how the portals work and try to reproduce it.

Instead of killing the Lady of Pain, why not just say that their project has succeeded? They don't need to kill the Lady because they've got their own Sigil, just as useful as the first one.

There are other clusters of portals they could use that wouldn't require them to use the City of Doors. It's hardly the only group of portals in the multiverse.

As for getting rid of the Lady of Pain, there are any number of ways. Perhaps her real goal is to escape Sigil, and she finally manages it. Maybe Aoskar is brought back to life and kills her. Maybe she has some sort of superior or predecessor (or a group of cosmic auditors a la Terry Pratchett's Reaper Man) who decides she's no longer fulfilling her duties in the way she was intended to, and destroys her/fires her. Maybe Rowan Darkwood kills her with the Sigil Spell. Maybe Tenebrous kills her with the Last Word. Maybe Vecna destroys her. Maybe she falls into despair and kills herself. Maybe someone forces her to experience all the pain she caused, and this destroys her or destroys her mind. Maybe the dabus rebel.

As I recall that Sigil-Model has it's own model of the Lady as well - what if, by virtue of modeling the real Sigil so closely, it has begun to actually produce portals approximate to the real Sigil as well simply by hitching a ride on the belief of Sigil... There may be an overlap between the two as the model starts picking up the same traits as the original. That can leave your PCs (his PCs really) an option to start breaking the system down later as well, and give the baatazu their own personal Sigil in Baator.

Belief has power. It's one of the cornerstones of the entire Planescape setting. The Lady is powerful, yes, but what if it's only because everyone believes it? The citizens of Sigil believe that She is powerful because the cage is safer with Her at the helm. Her enemies believe She is powerful because they have seen the flayed bodies, and heard the tales of the mazes, and they fear. Even the Gods believe that She is powerful because they know the tale of Aoskar and they feel the pain and antipathy from Sigil's portals. Everyone who knows of the Lady knows of Her as a thing of power, whether they like Her or not. Even those who would scheme against Her believe that they must be very careful in their work because to be discovered is to face Her power.

Now, if that all-pervasive belief can somehow be weakened, then perhaps the Lady herself would be less powerful. People would witness this weakness and word of it would spread. Doubt enters the equation. Eventually the Lady is mocked in the streets or defied by the masses, and She can do nothing. Worse still, eventually people might stop thinking of Her at all. She is not a concern. And that is when She might leave, or cease to be, or even cease to have been.

Belief is what the Planes revolve around. I think that in the right circumstances even the Lady might not be immune to that. . .

Basically, I agree with Sucros.

So maybe… she just does nothing about it. At all. No razor storms, no portal closures, no dramatic appearance. No appearances at all, in fact. She seems to do absolutely nothing to prevent the Baatezu “invasion” of her realm. Keep her motives and off-screen actions totally obscure, after all there’s no way the PC’s can check, right?

That preserves Her Serenity’s mystique and seeming omnipotence while still getting the result on the streets you want, and means you don’t have to ‘bring her back’ afterwards to rebalance things. If anyone questions why she seemingly isn’t doing anything (and many Sigilites surely would) just don’t explain. The Lady has always been elusive and enigmatic, beyond simple motives and easy understanding. It wouldn’t be the first time she abstained from immediate action either (I seem to remember she let loads of people worship her - for a bit - during the ‘Trolan Heresy’ of Harbinger House for example).

Of course Baatezu propaganda would shout loudly about their “destruction of the bladed dictator” and how they’ve got the Law on their side (the Guvnor factor, as described above), but that's for the characters to believe or not. Her Serenity was always a deus ex machina for the setting, so why bother making life complicated for yourself by showing a concrete reason. She does what she does; she’s the Lady of Pain.

She can step in to back the heroes up/save the day at the adventures climax with overwhelming force (again, like Harbinger House), which will - coincidentally - be the moment when all the pieces (like our treacherous Guvnor/pit fiend finally coming to the city) are in the right place for a decisive strike.

On the other hand, you could solve the problem this way...

Here’s a neat way of avoiding the problem of killing Her entirely: The Lady of Pain does nothing… because the PCs aren’t really in Sigil. The Baatezu “Sigil2” project is up and running: An exact duplicate of the Cage, complete with simulacra/doppelgangers/polymorphed fiendish actors in the place of major NPCs. The party thinks that their portal has taken them to the Cage, but in fact they’ve entered the simulation – either ‘accidentally’ (or at least by forces unknown to the Baatezu; maybe the Lady herself, who can do such portal tricks) or maybe deliberately as part of the Baatezu testing procedure.

The whole thing is in fact a dry run for the real event as in - geek trivia moment coming up - Doctor Who and the Android Invasion (look it up & all will become clear).

This way the whole plot unfolds exactly as you want it to, but carefully scripted by the Pit Fiends overseeing it all. PCs can notice the occasional oddity: A building doesn’t look quite right, an NPC knows or doesn’t know them when they should, they spy a couple of devils ‘fixing’ a broken Dabus or shifting the scenery at night, etc. Anyone familiar with the movie Dark City will also get the general idea.