Ragnarok

"Ragnarok. The end times. The destruction of the old world and the creation of the new one. Very few now know about it, and that is likely for the better. I see that look in your many eyes, Severed-One. It is because I remember every last detail, and I wish I did not"
-Saphira, patron deity of mankind
The War in Heaven, also called the Eclipse War and Rayg'nar-ak (Ragnarok), was a major conflict that took place toward the end of Arkon's Age of Myth. The war marked the end of the Late Dawn Era, and marked the beginning of the Tenebrous Era. 
The validity of the following facts is disputed, but according to the myths told on Arkon, the war was a cataclysmic conflict between the so-called "Good Gods" of the Reverent Pantheon and the "Evil Gods" of the Venerate Pantheon, which were lead by Axios the Divine and Angra the Betrayer respectively. The Pantheon's mortal allies were known as the Alliance of Light, while the Venerate's allies were known as the Seven Empires of Darkness, lead by the House of Arkon, the first demon clan. 
The war was said to rage for a thousand years, and only came to an end when Axios's chosen champion, whose name has been long forgotten, used the Marak-Ais Blade (now known as the Holy Blade of the Order of the White Cross) to defeat the Arkonic High Lord in battle. 
The war resulted in the end of high culture on Arkon, the crowning of Ava/Saphira as the holder of balance, the breakup of the Great Continent by Gaia's hand, and the beginning of the Tenebrous Era where the mortal nations, now free from the control of the gods as a result of Gaia's Law, fought each other into ruin. It was also the catalyst for the end of the mythical age.