Felthry's VGM Selection, long
Distorted Thoughts - Its Name is Ba'al
Revo
Bravely Default: For The Sequel & Bravely Second
https://www.youtube.com/watch?v=ZIQDuaw4tzs
First appearing in the trailer for Bravely Second, this theme is used when fighting Ba'als in both games. (two ba'als were added to Bravely Default as a teaser for the sequel)
The music does a fantastic job of being unnerving and creepy while at the same time driving. It starts off (after the obligatory disorienting start-of-battle motif) with a chorus of children singing in march-like time, accompanied by what seems to be a toy piano and drums, before the guitar and synths come in.
This interweaving of different instruments continues, as different melodies come in on different instruments, from guitar to synthesizer to string orchestra, eventually even including a proper piano to play a few dissonant notes before returning to the toy one.
Where this piece really shines, though, is the visuals that go with it. In game, this piece plays in an intensely strange situation: you've just entered what seems to be some kind of alternate dimension, where everything is pitch black except for a small scene of something--it varies with the ba'al, but it's often a scene that sharply contrasts the high-fantasy feel of the game; a laboratory, or ski lifts, for instance--in the background, while fighting something that can only be described as an eldritch abomination... while you have this piece prominently featuring toy pianos and small children's voices playing in the background.
It certainly gives a very eerie feel, which is exactly what they were going for.
re: Felthry's VGM Selection, long
One other note about the otherworldliness of the whole situation... All of the Ba'als fight in ways that no other foe in the game does.
One really interesting example of this is Ba'al ii; Redshirt, which... inverts HP so that you die at full HP instead of zero, and then relentlessly heals your party, with an attack that literally just consists of serving the entire party a nice dinner, complete with table and eating animations for the party members
Ba'al i; Turtle Dove also has an interesting mechanic in that it periodically causes party members to fall in love with other party members, which causes them to uncontrollably mimic every action the target of their affection takes... and if two people are in love with the same third person, if that third person does anything the other two will fight each other instead. And then it also has an attack that causes massive damage to anyone currently in love.