I thought it was made pretty clear that Dementors come from the ritual that Quirrel says summons Death, and that the True Patronus charm is the lost spell that banishes it. The other options seem more plausible, but really, I’d place my bet on some kind of poison that causes total and immediate brain death like Avada Kedavra does. I am not aware of any normal poisons that do that.
I thought it was made pretty clear that Dementors come from the ritual that Quirrel says summons Death, and that the True Patronus charm is the lost spell that banishes it. The other options seem more plausible, but really, I’d place my bet on some kind of poison that causes total and immediate brain death like Avada Kedavra does. I am not aware of any normal poisons that do that.