What Language Is This? (Darkest Night Text)

[Berethos]

It looks pretty much identical to the script written on the floor spell effects in the Thordan fight during his big screen-shatter attack.

As for what it actually is, my speculation is that it is the Eorzean script (or something meant to resemble it but isn't it, which I personally think is more likely) with a bunch of extras added to it for artistic license, possibly to make it appear to be an ancient or esoteric form of writing used specifically for spells (which would be consistent with the examples where it has thus far appeared).

I wouldn't be surprised to find it visible in a variety of other spell effects if one looks carefully, but it's definitely appeared in at least one other place that I can immediately remember.

I tried to make sense of it when I first noticed it in the Thordan fight, but with so many of the letters ranging from slightly to different to not existing in the Eorzean alphabet, I don't think it's decipherable with the information we currently have.