Complex sentence: The final blow was a severe drought in the region that resulted in a drastic drop in precipitation between 2200 and 2150 BCE, which in turn prevented the normal flooding of the Nile.
Split into simpler sentences: The final blow was the 22nd century BC drought in the region that resulted in a drastic drop in precipitation. For at least some years between 2200 and 2150 BCE, this prevented the normal flooding of the Nile.