Everything was peaceful until the Ancients started hearing a "Sound" from beneath the center of the earth, a sickness of the planet. The Sound distorted the creation magics that the Ancients wielded, eventually resulting the Final Days as their magics spun out of control. In a last ditch attempt to save themselves, the surviving Ancients sacrificed half of what remained of themselves to summon Zodiark, which rewrote the laws of reality on the planet to prevent the use of creation magics. But the Source of the Sound was not fixed and as of 4.5 it's starting to come back again, which magic going haywire. Worse, is that the Sundering of the planet into 14 pieces hasn't stopped the Sound, as in 5.2 the Scions are hearing the Sound even while on another shard.
In 5.3 we found out that during the summoning of Zodiark, one of the convocation members apparently got screwed up, probably by the Sound. Said convocation member is almost certainly Fandaniel. Zenos wants to recreate the Final Days, and he's now working with Fandaniel. It seems pretty obvious that 6.0 is going to culminate in us confronting the Source of the Sound.
One of the reported trademark names for the 6.0 FFXIV expansion is "Forspoken", which means to curse or enchant or bewitch something or someone. That'd line up with 6.0 being about the Sound and its effect on the planet. It's even addressed as a sickness by Lahabrea in 2.0.