Tiamat and Bahamut have been rivals since the beginning of time. In the lore, they are supposed to be complements of each other, similar to the idea of yin and yang. However, their personalities were too strong and they never reconciled their differences.  When Tiamat and Bahamut separated after the shattering of the First World, they amassed their followers who were also in competition with each other. When violence broke out between the two factions, tensions between the siblings rose to a fever pitch.