A friend I know IMed me two weeks ago with a problem with his new campaign. And what a problem it was.
Him: "I have this great campaign, but I need a way to kill the Lady of Pain."
Me: "You can't kill the Lady of Pain. Planescape doesn't work like that."
Him: "But I need Sigil to be under the control of the Baatezu, and there's no way the Lady of Pain would let that happen."
Me: "You can't kill the Lady of Pain. You just can't."
Him: "But my entire idea revolves around t he Baatezu using Sigil to attack other planes."
After a little more dialogue and a lot more begging, I agree to see if I can think of anything. Which, as usual, means search Planewalker.com for relevant info. Sadly, I couldn't find anything, so I had to actually think. After a while, I finally came up with an idea that I like more and more the more I think about it.
I wanted something a little more realistic than "The Lady just got up and left" and a little less cliche than "Super powerful diety gathers enough power to kill the Lady." So, here it is....
A Baatezu General (or any race for that matter, but they need to be organized in a highly efficient army) sends the majority of his army into Sigil. The demonic army than sits down, and prays to the Lady of Pain. The Lady cannot maze or kill the entire army, and they are able to pray unhindered (though I'm sure the Lady isn't going to allow this without killing as many as possible, but when you're a demon, nobody cares).
What happens next is open for discussion. I have to assume there is a good reason why the Lady kills any would-be followers. A catastrophic explosion is which the lady is destroyed and takes a good portion of Sigil with her is my favorite option. However, it's really up in the air what you want to have happen next. If you really want, another major power could decide to claim Sigil (that power could even give the idea to the demon general). After that, the chaos begins.
I think it could work, and it's defenitly never been tried before. What does everyone else here think?