Final Fantasy III is an old game.
Like most old games back in the day, this means that most of the background information and lore was confined to guidebooks or magazines or what have you. Very rarely was any of this info ever brought over westward, with the guides and whatnot removing it. And if the game in question never released in the west...
At the time of this writing, I do not have these materials nor the means of which to read them. My info instead comes second-hand, as a JP wiki does make mentions of the info and JP twitter users also occasionally discuss these faccets. For those who wish to verify or help, the guides in question are NTT publishing guides. At least two of the guides are floating online, but neither of them have been translated and the one that has most/all of the lore doesn't seem to among those.
Meanwhile, the 3D remake had its own minor set of additions and changes to the lore... most of which is hidden in the unused text...
Magic in the FF3 world had a surprising origin.
At some point in time, Doga gave humanity the gift of magic. Humanity used magic in various ways to aid in living or for religious purposes. Magic in those days did not have specific designations; It was all simply magic.
At some point, however, Sasune went to war with Saronia. While the countries started off as relatively evenly matched, Sasune started to research Doga's magic.
A rough + machine translated version of the Guide's telling follows:
あまご きとう ぎしき 魔法の成立はそもそもが宗教的 であった。雨乞いや祈祷の儀式に おいて使用されたのに始まり、や がて病気の治療に使用される程度 で、白、黒といった分類は存在し なかった。魔法がその歴史において二極化 を見るのは2つの大国間において 起こった戦争に起因する。
スーン王が攻撃手段の1つとして まどうし 魔法に着目し、国内の魔道師を配 さっしょう 下に置き、殺傷の手段として産み 出されたのが黒魔法の始まりであ る。だが、一部の魔道師は魔法を 殺傷の道具として使うべきではな 「いと考え、サスーンを脱出、サロ ※ニアへと移り住み、新たなる魔法 S の体系を確立した。これが白魔法 の始まりである。やがて黒魔法の せんきょう 発達とともに、戦況はサスーンに 有利となり、多くのサロニアの兵 士たちが黒魔法によって傷つき、 倒れていった。事態を重く見たサ ロニア王は白魔道師たちに救いの 手を求める。ここで初めて白魔道 しろまどうし 師は黒魔法から人々を守るという たいめいぶん 大義名分のもとに戦争の渦へその 身を投じることになるのである。 しかし結果的にはそれが裏目と なり、次々に互いの魔法はその威 力をエスカレートさせていった。
さつりく そして遂には究極の殺戮魔法とし て白はホーリー、黒はフレアの完 成をみる。だが、その時誰一人と してこの魔法の本当の恐ろしさに 気がついた者はいなかった。そし て悲劇が起こった。その互いの究 極魔法の巨大なエネルギーがぶつ かりあった瞬間、両軍の数万人の 兵士が一瞬にして消滅してしまっ たのだ。この戦いを機に戦乱は一 気に終結へと向かう。そして戦争 の終結とともに大きすぎる魔法の 力は禁断の地「エウレカ」へ封印 されたのである.
The establishment of magic was religious in nature. It was first used in rituals to pray for rain and to cure illnesses, and there was no such classification as black and white. The polarization of magic in its history can be attributed to the wars between the two great powers.
King Sasune focused on magic as a means of attack, placed domestic mages under his control, and produced black magic as a means of killing and wounding, which was the beginning of black magic. Some mages, however, believed that magic should not be used as a tool for killing, so they fled Sassoon and moved to Saronia, where they established a new system of magic. This was the beginning of white magic. With the development of black magic, the war situation became favorable to Sassoon, and many Saronian soldiers were wounded and fell by black magic. The Saronian king took the situation very seriously and asked the white magicians for help. Here, for the first time, the White Mages throw themselves into the vortex of war in the name of protecting people from black magic. However, this backfired, and one by one their magic escalated in power.
Finally, the ultimate killing magic, Holy for the white and Flare for the black, was completed. But at that time, no one realized the true horror of this magic. Then tragedy struck. When the two armies collided, tens of thousands of soldiers on both sides were instantly annihilated. This battle brought the war to an end. And with the end of the war, the too great power of magic was sealed away in the forbidden land of Eureka.
Within the Final Fantasy III Perfect Analysis first volume are numerous references to Dark Knights. In particular, a recurring story is that Dark Knights used to serve Saronia alongside Knights. This continued until one day where the King of Saronia at the time grew to fear their power. He promptly sealed the blade of one particular Dark Knight, who promptly left for the mountains to practice his craft. As the name of this topic shows, the Dark Knight in question is named "Leonhart" after the ''Final Fantasy II'' character of the same name.
The relevant text is as follows:
通常兵器を使いこ なせるキャラクター と言えば戦士系であ る。ただ、同じ系列 でもナイトと魔剣士 の剣に対する考え方 には大きな違いがあ る。剣が持つ力の二 せい 面性のうち、正の力 い を活かした剣士がナ イトであるとすれば、 負の力を活かした剣 い 士が魔剣士なのであ る。ともにサロニア 王室直属の優秀な剣 士だったが、力の源 りょうきょく フォース みなしと ふういん はその両極にあった のだ。負の力をおそ れた時の王は、魔剣 士レオンハルトを追 放、暗黒剣を封印し てしまった。追放さ れたレオンハルトは サロニアの南西に居 を移し、暗黒剣の道 を極めていった。こ の地が後のファルガ バードである。
Characters who can use normal weapons are warriors. However, even within the same type, there is a big difference in the way that knights and magic swordsmen think about swords. If a knight is a swordsman who uses the positive power of the sword, then a magic swordsman is a swordsman who uses the negative power. Both were excellent swordsmen directly under the royal family of Salonia, but their sources of power were at opposite ends of the spectrum. The king at the time, fearing the negative power, banished the magic swordsman Leonhardt and sealed away the dark sword. After being banished, Leonhardt moved to the southwest of Salonia and mastered the path of the dark sword. This place is what would later become Falgabard.
ちょう しょう 世界最大の都市サロニアは、文化の象 徴の図書館がある街、ナイト系民族の 街、魔剣士系民族の街の、3つに分かれ ていた。しかし、年々魔剣士一族は追放 により減少し、ナイト系の人だけが住み つくようになった。その中の1つ、 その中の1つ、南西 の街は工業都市。工場で働く人々のため に、酒場も経営されている。
The world's largest city, Saronia, was divided into three parts: the city with the Library of Culture, the city of the Knight tribe, and the city of the Magic Swordsman tribe. However, the number of the Magic Swordsman tribe decreased year by year due to expulsion, and only the Knight tribe came to live there. One of these, the city in the southwest, is an industrial city. There is also a bar run for the people who work in the factories.
サロニアから移り住んだ、暗黒剣を使う一族の村
魔剣士の隠れ里、ファルガバード。サ ロニアから移り住んだ、暗黒剣を使う一 族の村である。しかし、負の力を使いこ なせる者は少なく、特に魔剣士と呼ばれ る者は、わずか数名しか存在していない。 しかも、そのうちの大部分が高齢である ため、その使い手として後を継ぐことが できる者をひたすら待ち続けている。
A village of dark sword wielders who migrated from Saronia
Falgabard, a hidden village of magic sword wielders. A village of dark sword wielders who migrated from Saronia. However, there are few who can use negative powers, and there are only a few people who are called magic sword wielders. Moreover, most of them are elderly, so they are waiting for someone who can succeed them as wielders.
Or "The Time When Light Was Bad, Actually"
In the times of old, the Ancients grew arrogant. In their arrogance, they decided that there was no risk in only using the Light and modeled their society around it.
But there was.
By unsettling the equalibrium, the Ancients unwittingly unleashed a Flood of Light which threatened to destroy the entire world. Meanwhile, in the World of Darkness, the Dark Crystals chose four warriors to become the Warriors of the Dark. The Dark Warriors would eventually travel to the World of Light and confront the Cloud of Darkness, defeating it and saving both worlds...at a great cost.
The Wrath of Light is a pure background element and, this being a Famicom-era game, the game itself gives no proper explanation to it. What's known is that the ancients made an oopsie, the Warriors of Darkness had to clean up their mess and that Owen cryogenically froze Desch so that he could protect the Tower of Owen in case something happened [1].
The remake adds slightly to the nature of the Flood of Light, mostly due to expanding upon the nature of Xande's Flood of Darkness. The Flood of Darkness is said to have cut the world off from the sun, creating an eternal night and causing time to stop. The implication is that the Flood of Light did the same thing: The Ancients created an eternal day and it threatened to freeze time entirely.
The unused text of the remake heavily implies that this was indeed the case, but adds a further wrinkle onto the story; There's an implied connection between the Earth and the Sun in the FF3 remake. This is primarily mentioned by the Warriors of Darkness, but Desch and Doga both explain it more properly. In particular, all three mention an odd quirk: Prior to the Flood of Light, the Sun used to revolve around the Earth. During the Flood, the Sun stopped moving. After the Warriors of Darkness saved the World, the Earth now revolves around the Sun.
The concept of a "Flood of Light" was also used in Final Fantasy XIV's "Shadowbringers" expansion. However, it lacks the timestop/"Earth and the Sun" elements, leading to a different interpretation. Notably, while there is no sign of the world having been scarred by the Flood of Light in III, XIV's the First was reduced to a single region due to all other places being erased by the Flood.
[1]: Even then, the remake recontextualizes this by having Desch be frozen long after the Flood of Light.