Primals aren't even Primals. For example, Titan, Ifrit, and even Bahamut are not real. They're Aetherial manifestations of an image made by faith. Basically a being or group of beings use their faith and channel it into a crystal to manifest a primal. The beastman tribes did this with Ifrit, Garuda, and so forth. Yasyle did it with Shiva, and the Allagans did it with Bahamut.

The Warrior of Light is blessed to fulfill a task for the light when Darkness is out of balance. The Warrior of Chaos/Darkness appears when the Light is out of balance. That was pretty much explained in FFIII and has been the cornerstone of the numbered franchise (not sure if FFT, FFTA, ect have WoL/WoD).