My Planescape campaign ended with them all dying.
I didn't plan it that way. They were heading into the Abyss to deal with their main nemesis, a demon-dragon, and recover a powerful plane-altering artifact that it had acquired.
If they'd planned properly, they'd probably have survived. They did managed to kill Lachazdaervirak - the dragon - but only the enchantress was still standing. Lacking any suitable escape plan, she was then killed by demons.
However, they wre ultimately successful - the dragon had hidden the artifact well, and no-one else ever managed to find it. By killing the one person who still knew where it was, the party nullified the threat...