Bahamut was once a mortal dragon-born, born roughly about 710 B.B and almost
instantly sent to be raised as a Paladin of Pelor. Bahamut started a new era known as "After Bahamut" (abbreviated as A.B) after becoming the first mortal to naturally ascend to godhood. He also started a following of good-aligned dragon-born known as The Knights of Bahamut.
Bahamut was born roughly 710 B.B to devoted worshipers of Pelor. As a sign of their faith, they offered their child to the order of Paladins to be raised to smite evil in the god's name.
Bahamut rose in power as he aged, becoming highly skilled in both martial prowess and also divine power, often being considered a militia's saving grace in times of great peril. In his time in the order, Bahamut rose the ranks to becoming Captain of his own squadron.
During his time in the order, his sister, Tiamat, went down the opposite path. Becoming a vile and twisted being, corrupted by evil. Tiamat rose in power, eventually controlling a small army of her own.
The two armies would eventually clash at a site just north of Cawic (What would later become The Temple of Elemental Evil.) in a clash known as "The Battle of Dragon's End" at which Bahamut slew his sister.
As an offering for the constant devotion and the sacrifice he had made, Pelor offered Bahamut something that no other mortal had ever dreamt of: Godhood. Bahamut became the God of good Dragons, Wisdom and the Wind (Relating to the Draconic wings he had sprouted earlier in life.), taking his place among the other gods in Arborea.
Upon visually seeing Bahamut ascend, the remaining mortals told many of the sight they had seen and it quickly spread that it was indeed possible for mortals to obtain godhood. Many began their own campaigns in order for them to achieve such a feat themselves.
This sharp spike in adventuring heroes saw a new era be named "After Bahamut". (as in after Bahamut ascended.)
Bahamut also gathered a vast following, forging a faction of knights that were devoted to stamp out evil in all forms, they were named The Knights of Bahamut.