Source War
No one knows who started the war, but the good gods who honored the Vow of Equity sided with each other and the evil gods who broke it fought back.
As a result, a great war broke out between the two sides, and by most history books today, the two sides are called the Benevolents and the Recreants, names that still hold true today
Many great, terrible monsters were born in this era
Most of these monsters are said to be extinct today, either destroyed by the Benevolents or weakened to a state that they have decayed over time
Source Relics were extremely powerful things created during this time to aid in the war - weapons, equipment, energy sources, etc.
Note: items created before the Source War are called Archaic Relics, or Arch Relic… more rare than legendary magic items, grade X
During the war, the Benevolents saw the strength of mortals when they were able to banish Tiamat with the combined power of civilizations
The war was put to an end when Ao himself descended upon the realm and banished all other-worldly creatures, rending various injuries to all the deities that had gotten involved and sending them back to their home planes. Ao placed the Divine Order that prohibited gods and goddesses from directly materializing in the prime material plane.