What if the civil war that took place in Atmora was The needs against the Atmorans and the Skaal. The Skaal claim that the allmaker was once worshipped by ancient nords and some people beleive that the Nedes originated in Atmora So why not all to be true?
Civil War in Atmora? I believe there is no historic record of such thing. Actually, little is known about Atmora other than it being a cold, antarctic-like continent that was once the motherland of the race that originated the Nords.
According to the book Frontier, Conquest an Atmoran civil war was what prompted Ysgramor to sail to Tamriel, i don't think Nedes vs Atmorans would be called a civil war (even if they were once the same they would be too far removed).