I don't have any historical baggage with the setting, so when it was described as Sigil and the Outlands I believed them. I will also say that when I did a little wiki/google research when they announced the setting, almost all of the information I found about Planescape was about:
- Sigil (vast majority of the info): city, factions, history, etc.
- The Lady of Pain
- Outlands
I really don't remember much info about the planes other than Sigil connected to them all. And maybe the blood war and the modron march have planescape origins?
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