What is Foul?

[Iscah]

I think Flare makes more sense as the anti-Holy - both learned at Lv50, and paired up as RDM's Verflare/Verholy finisher spells.

[Floortank]

i don't think we would get a dark upgrade without having lower level dark spells

Sure we could. Holy isn't an upgrade of any particular unaspected damaging spell. Holy is very similar to Foul.

[Jonnycbad]

I think Flare makes more sense as the anti-Holy - both learned at Lv50, and paired up as RDM's Verflare/Verholy finisher spells.

Flare is learned at level 50, Holy is learned at level 45. According to the Lore book, Verholy and Verflare don't share THAT much in common with the original spells other than being namesakes.

Flare: Shanttoto the matriarch of black magic, derived this spell in her reckless pursuit of ultimate destructive power. Combining one's own mana with ambient aether, the caster triggers an explosion of monumental force.

Holy:This incandescent blast of purifying energies was devised by the white mages of Amdapor to combat the voidsent servants of Mhachi sorcerers. The battlefields of the War of the Magi were often illuminated by spell's distinctive explosions

Verflare: Its name is a portmanteau of "vermillion" and "flare". This incantation seeks to rival the destructive power of the black mage spell that shares the latter's denomination. Rather than drawing on ambient aether to cause a single, massive explosion, Verflare overlaps a number of smaller arcane eruptions.

Verholy: An adaption of the forbidden white mage spell "Holy", this purifying blast was conceived as a means to banish the lingering voidsent, which were summoned during the War of the Magi

So Flare combines the user's mana with ambient aether to make a gigantic explosion. Holy is a purifying blast of light energy to wipe out low level voidsent. Verflare uses a chain of small microexplosions instead of using ambient aether to make a localized magical burst. Verholy is a singular blast of "holy". It's more concentrated and the description says it was used to pick off individual voidsent survivors after the war, as opposed to the WHM version which was probably used against hoards/armies.

[Jonnycbad]

Sure we could. Holy isn't an upgrade of any particular unaspected damaging spell. Holy is very similar to Foul.

To be fair, White Mages technically can cast "Banish" and "Dia" but the player cannot. These were spells that used to be in 1.0, but were removed around 1.2. There are several White Mage-type mobs (Light Sprites/Holy Sprites/Amdapori statues etc) that still cast these spells, and it's likely that Holy is to Banish what Flare is to Fire. It's possible the original Amdapori White Mages specialized in pure White Magic. The player White Mage is technically is a hybrid of Conjurer/White Mage, since White Magic is a lost art, the core of our skills is based on Conjury (elemental magic), hence why we have Stone/Aero/Water instead of pure light magic like Amdapor white mages did with Banish spells.

[Kalise]

To be fair, White Mages technically can cast "Banish" and "Dia" but the player cannot. These were spells that used to be in 1.0, but were removed around 1.2. There are several White Mage-type mobs (Light Sprites/Holy Sprites/Amdapori statues etc) that still cast these spells, and it's likely that Holy is to Banish what Flare is to Fire.

The one thing that throws a spanner in the works of this idea, is that Holyra, Holyga and Holyja exist in the series.

Which is different from the other "Ancient Magicks" which didn't have that sort of upgrade system, and the closest example is Bahamut having Gigaflare and Tetraflare which share the "Flare" name (Though, were neither Fire damage nor shared any other similarities with the Flare spell in the game)

So while Banish > Banishra > Banishga > (Banishja >) Holy would make some level of sense as other elemental types, it doesn't suggest that with the fact that upgrades to Holy exist within the series.

Much like how Foul could be the capstone spell for the Dark line instead of the Dark version of Holy (Unholy Darkness, Dark Holy or Mega White depending on which iteration of the game you look at... Rabanastre raid uses Unholy Darkness, Tactics called it Unholy Darkness or Dark Holy and Mystic Quest called it Mega White)

Meaning that there's a possibility that both Banish and Holy could actually both have a unique capstone spell to be the powerful alternate named "Ancient" spell. (I have to put Ancient in quotation marks because technically, everything we learn as SCH is ancient because it's learned from the Nymian scholars through Lily/Surito. As is everything we learn as BLM most of which is learned through the book of Nald'thal. As is everything we learn as SMN which is learned through ancient Allagan texts/Prin.)

Especially if you also look at the description you provided regarding Holy. If Holy was to be for dealing with the lower ranking voidsent servants... You'd think the Adampori would have a much more powerful spell for nuking the higher ranking voidsent that would likely be employed during the war. Possibly something like Nova?

[ObsidianFire]

There's also the issue that in the case of White Mage, a lot of the higher level White Magics are probably illegal for even us or the Pajali to use because they are what led to the land being drained of aether. Black Mages doesn't really care all that much about that and so they just go ham with crazy powerful magic use. The Elementals are a natural check on the White Mage job and we know there are certain things White Mages were capable of doing that the Elementals don't want to be discovered.