Originally Posted by Argyle
I never understood the origins of the mindflayers. Are they naturally from another plane?

The Forgotten Realms Wiki talks a bit about their origins, on the Mind Flayer page, History section. There are only theories, it seems.

Originally Posted by Artagel
Tiefling Quest in BG2 went to Sigil, I believe.

The troupe themselves come from Sigil, but if you mean the ingame quest, no. The quest takes you to an extraplanar prison.

"We make our choices and take what comes and the rest is void."