What is Foul?

[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]

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?