Ok, now i need some help with this, it's quite important for me.
Imagine some Abyssal general opening portal to Sigil (this is hardly possible, i know, but try to survive it...=)), now hordes of Tanar'ri flowing to Sigil, killing'n'destroying everything how they're happy that they finally got there. Lady of Pain dies in attempt to stop them, baatezu got information about this all, so they concentrate all their forces to get to the portal, and at the end, they make it to Sigil too, so Blood War goes there...=)
now, i have two questions:
1. about layers sliding to another planes: what does this depend on? if i got e.g. Elysium, and i get a hordes of baatezu / whoever there and get all the native Elysium people killed (or most of them), then ruling belief on the plane would be lawful evil, so the plane itself should slide into baator...or am i wrong? does only native people's belief matter?
2. now, which power would be able to defeat both baatezu & tanar'ri & hold sigil, and then, would be willing to leave it as soon as it's "fortified" again? probably no... i thought that General of Gehenna could, maybe, for xchange of something? & there are those "prophecies" that tells that he'd be able to finally end Blood War...
or, i thought about Apomps...? he's evil, yes, but... i don't think, that he would let the yougoloths to have the city, and i doubt that yougoloths would still remain "neutral"... i think that they would try to get the city for themselves, as it would seem to them, that they're loosing control of baatezu&tanar'ri...? or am i wrong? i think, as soon as city's defences would be breached, and rumour about it would spread, everybody would want the city for himself... so now what? the Greatest Battle of All Good against All Evil? i'd like to avoid that...
any ideas & fabulations & informations warmly welcomed...
hehe
With regards killing The Lady of Pain... I'm not sure that it is possible short of destroying the city of Sigil. She's a mystery, enigma an unknown basically. Entire libraries are filled with tomes speculating on who and what she is... and pretty much all of that information is irrelevant and baseless guess-work.
All that we know about her is that the Dabus are her servants, when she appears her shadow flays to death anyone it passes over. As well as the fact that hose who grow to sufficient strength in the City to threaten her - they get Mazed in to a mini-prison-demiplane on the Ethereal. She also controls the portals to Sigil and is believed to be the reason why Deities and Powers are unable to enter.
So the question is... how do you kill her? There have never been any stats published, she does not adhere to the rules of a Deity (in fact she flays any that attempt to worship her).
With regards to the Baatezu and Tanar'ri bringing the Blood War to Sigil - it's happened in the past (at least once). After several weeks of fighting all that remains is an area of the Hive ward called The Slags that is still dangerous to travel through.
If The Lady of Pain controls the portals - then any invasion is thwarted by her simply shutting down specific portals or in fact all of them. The only way in or out of the city is through Portals, gating in reinforcements or other means of entry simply do not work.
With regards to killing all the natives of Elysium... the plane has a property called Traveller's Travail (if I remember correctly) that basically means that the length of any journey is determined by how much good a person does along the way. If you help a woman find her lost son - your journey will go faster. Ignore her plight or commit an evil act and you'll not be going anywhere soon.
Then there is the fact that the plane is infinite to take in to consideration and that it has more than one layer. On the other hand the number of fiends is also infinite and so it is conceivable that the fiends could spend an eternity in an organised search (the Baatezu at least) trying to kill of every native of Elysium.
Planes or portions there-of generally shift when large groups of people residing in those locations shift in their alignment and how they behave. Gate-Towns are a good example, but there are also others.
The criteria or threshold that needs to be reached before a slide occurs is entirely up to the GM. So is killing off The Lady of Pain if you wanted to. However, in my opinion it removes one of the fundamental parts of the campaign setting.
Those are my thoughts.