On a note dealing with Fandaniel, I had a random thought which may explain what's going on with him. It deals with the Ascians we've defeated so far and our fights against them. Each one of them has fit the Scion of Darkness motif in either their actions or their fights, with as of right now, Fandaniel being the outlier (mostly due to screen time).

Lahabrea's Scion of Darkness title is "the Corrupt". From his actions of teaching and encouraging the beast tribes to summon their gods, he is indeed a Corrupter.
Nabriales is "the Impure", and the fight against him involved not only moves that looked wholey unnatural with tendrils of darkness, but he also had a "reverse" realm where the colors were inversed, coming across as completely unnatural. He indeed fits Impure.
Igeyorhm is "the Whisperer", and was the main cause of the 13th being overcome by too much darkness from teaching its warriors dark arts, whispering to them its secrets. In a sense, she fits "the Whisperer"
Emet-Selch is "the Death Seraph", and has not only helped destroy numerous shards, but turns into the literal embodiment of death in his final battle. He definitely fits the title.
Elidibus is the "Keeper of Precepts". Precepts are general rules meant to regulate behavior or thought. He kept all the other Ascians in line with his rules in order to prevent another incident like with the 13th, so he too fits the title.

Now we come to Fandaniel, whose Scion of Darkness title would be "Bringer of Order". Notice instantly the problem? He's not bringing order at all, but sheer chaos, and takes delight in it. This would be the opposite of what his Scion of Darkness title is. If I were to make a wager on what's going on, he is likely corrupted by that which caused The Sound. This would explain why he had to be kept in line so much by Elidibus, and why he's perfectly willing to destroy the world by recreating The Final Days, something that would be the complete opposite of what the Ascians want. It's very likely his death or his plot will result in the return of The Sound's creator.

It was mentioned earlier that there may be FFX vibes in this expansion. If so, then expect The Sound's creator to be a Yu Yevon expy, something that possesses others and corrupts them into doing what it desires, which is apparently full on destruction of everything.