Originally Posted by
Plegethorn
I've actually been thinking on this topic recently, and I have a theory, if you guys are willing :3
So, drawing inspiration from older FF games, I'll begin.
In the older games (mainly 3) Warriors of Darkness are called when the Light in the world becomes too strong and threatens to destabilise it. They are usually chosen by the crystal to bring a balance. Sometimes working with the WoL, sometimes not.
It is my thought that Eorzea is becoming unstable due to the amount of darkness that the WoL has actually banished. So with that in mind, the mothercrystal felt need to interfere, lest you destabilize the world.
Another idea focuses on FF4.
In FF4, there existed Crystals of Light and crystals of Darkness. What if the Ascians actually hold the Crystals of Darkness? They could plead with them for a Warrior of darkness, possibly by claiming that light was threatening to destabilize the realm. Said Warrior would then start doing things to introduce Darkness back into the world.