Strictly speaking, Gilles de Rais wasn't a magus. During his life, he was a marshal who fought on the battlefield and not a user of magic. He charged into battle as a warrior. Even if Gilles was summoned as a Servant, his class should have been Saber rather than Caster.
However, for Gilles, Jeanne meant almost everything.
After Jeanne's capture, the mental blow to the marshal was substantial. He retreated to the territories of Machecoul and Tiffauges, underwent a drastic change in personality, and delved into alchemy, hoping to uncover the secret of the Philosopher's Stone through blood. Eventually, he tortured and killed over three hundred children. It's clear that Gilles suffered immense trauma after Jeanne's death.
Given this, Gilles fell into darkness, becoming a murderer, cursing his country, the world, and even the deities he once believed in. He became a full-fledged heretic.
While Jeanne was unjustly accused and burned as a witch, Gilles' deeds were deserving of punishment, leading to his execution.
However, Jeanne's death was the catalyst for all of this. Regardless, this is undeniable.
Now, Gilles' manifestation as a Caster class Servant demonstrates this.
This is Gilles' appearance after he descended into darkness, becoming a madman, cursing the world, his country, and the gods.
The marshal Gilles de Rais, who led the French army, hadn't fallen into madness to the same extent.
But the Caster before them was different.
He had fallen into madness.
He had tortured and killed over three hundred children.
He had attempted to unlock the secret of the Philosopher's Stone through blood, but without success.
Therefore, he would slaughter, he would destroy, he would embrace cruelty and bloodshed without turning back.
That's why Jeanne felt sorrow.
"Gilles..."
Confronting Caster's frantic state born from her death, Jeanne knew she couldn't persuade him to turn back.
Heroic Spirits are born in the past and die in the past.
This Gilles de Rais before her could no longer be redeemed.
All Jeanne could do was face this sin and ask a question.
"Why are you helping the 'Dragon Witch' commit such crimes?"
Facing her former friend, Jeanne couldn't help but question him. After all, this is the country they protected. This was their shared battlefield, their spiritual sanctuary.
"You've tirelessly protected this nation. Are you really willing to destroy it?"
This was the only question Jeanne could ask.
But...
"...You're still as kind as before. Even after death, you've never hated this country. Truly, you are the Saint we believe in, the one chosen by God."
Suddenly, Caster's frenzied tone disappeared, replaced by a calm demeanor.
However, this calmness was only on the surface.
The hatred and darkness that burned within Caster couldn't be quelled.
After all...
"This isn't a crime; it's our rightful duty, Jeanne!"
Caster spread his arms and shouted with fervor.
"You're right!"
"We've guarded this nation!"
"We've fought for this nation!"
"If we were absent, this nation would have perished long ago!"
"But... but but but but but...! It betrayed you! Betrayed me! Betrayed everything we've done!"
Caster's deceitful words dripped with hatred.
"If that's the case...! If that's the case...! Then we should punish this corrupt, erroneous, and foolish nation in the same way!"
"Absolutely!"
"This is punishment, not a crime! The birth of the 'Dragon Witch' proves that!"
"She's your dark side, a force meant to seek vengeance upon this nation!"
"This is how our esteemed Saint awakens from her blind devotion, reclaiming the rights she should have!"
Caster then let out a sorrowful cry.
"In the end... in the end, why do you appear? And why do you stand against us!?"
"Is this the curse of God?! Is this the proof that they won't let you go even after your death? To turn the Saint into a witch and use her to seek revenge on this country! To summon the Saint who protected this country and sit high above, watching us all slaughter each other! Is this your entertainment!?"
"I hate it! I hate it! I hate it! I hate it! I hate it! Aaargh!!!!!"
Caster screamed while clutching his head, and his uncontrollable magical power exploded.
Under the influence of this surge of power, the sea monsters led by Caster also began to writhe, seemingly caught up in a frenzy. They launched themselves from the ground one after another, hurtling toward Rozen and Jeanne.
"Danger!"
Jeanne immediately leaped in front of Rozen, holding up her Holy Flag to prepare for the incoming sea monsters.
However, it was Rozen who pulled her back.
"No need to fight. Our goal is to retreat; prolonged battle won't be in our favor."
As he said this, Rozen controlled his familiar beneath him, fully activating his Heaven's Eye.
In this state, Rozen could clearly see the trajectories and directions of the approaching sea monsters.
"Take to the skies!"
Rozen piloted the massive eagle, soaring upward rapidly.
Faced with the sea monsters hurtling toward them like projectiles, Rozen used his Heaven's Eye to anticipate their attacks. He deftly maneuvered the eagle, avoiding them with a series of evasive maneuvers that left behind afterimages.
Seeing this, Caster regained his composure.
"Jeanne... Saint..."
Caster's eyes were focused entirely on Jeanne; he murmured as such.
"Even you, I won't let you hinder her..."
Leaving those words behind, Caster raised the Spellbook in his hand.
Ultimately, despite Caster's research into alchemy during his retirement, his efforts to unlock the secret of the Philosopher's Stone weren't enough to gain a Caster-class Spirit Origin.
Caster's possession of this Spirit Origin was entirely thanks to the Spellbook he was using, the "Prelati's Spellbook."
This was a Spellbook capable of summoning and commanding sea monsters, those sea monsters recorded within it. It contained forbidden knowledge about ancient evil deities that existed before humans.
Although Caster held only the Italian translation, the original text was written in ancient Chinese from the Xia Dynasty. It's said that the even earlier source was recorded in a language beyond human understanding. If one traced its origin, it could be dated back to around 3,000 years ago.
Thus, even with just the Italian translation, the book retained exceptional power. It could be used to summon and command sea monsters and even serve as a magical reactor, ignoring the caster's own magical energy to perform grand rituals and ceremonial magic.
Its rank was A+, belonging to the category of anti-army Noble Phantasms.
Currently, Caster was employing this book to command a legion of sea monsters, pursuing the giant eagle flying away.