.... you can ALSO get spontaneous tieflings from major events, like you mention - the Elturel incident did indeed cause many spontaneous tieflings when it occurred; people unwilling transformed by the energies of the planes. There would also be some who were preyed upon and convinced to make bargains, during the time they were trapped there - in particular, scared citizens who would latch onto any promise of safety for fear of their lives.
We don't actually know what numbers of the refugees from Elturel were born tieflings, and which became so suddenly through accident or deal; short of asking each one individually for their story, we have no way of knowing... and chances are any who *were* tricked or tempted into deals that made them this way won't want to talk about it.
+1
thanks for sharing I did not know about this the Grove makes a bit more sense now.
