Hagiography of St. Leo of Monte Cassino

What exactly makes a human something more? How can a simple human stop being human and become something greater?
Ancient masters of The Ascending Path would give a detailed description of the way one has to follow to ascend; the necessary rites, offerings, and hundreds of cryptic writings with scraps of the true answer hidden behind ancient codes.
The Forbidden's answer would be very similar, but adding human sacrifice, books bound in the skin of innocent children, and entire treatises on 'discarding the limitations of morality and humanity' - which is why most want them dead.
Awakened prowling blood soaked battlegrounds of Europe and Asia don't know the answer, despite successfully transcending human limitations. To them, 'becoming something more' was merely an accident. A random (or God's, depending on whom you ask) throw of dice.
And then there is an answer of the Church.

Summary

All details of the life of St. Leo of Monte Cassino hail from a single, heavily censored book - the "Hagiography of St. Leo of Monte Cassino", whose exact writer decided to remain anonymous. It was purposefully saved when the Church rewrote the past using the Hishbon Aet, (Hebrew, the Engine of Time), essentially preventing the War of Wrath from ever happening.
The censorship is understandable: the details of the mysterious archenemy of the Church (and Earth as a whole) were purged from history with extreme prejudice. Only this legend gives ANY information on them. It is censored heavily, to make sure that only the bare minimum of knowledge will be betrayed.
The legend begins with St. Leo's early life. He was far from a paragon of virtue. A minor noble from Italy, born around 1325, a child of the cursed generation raised up when the War of Wrath began. He fought in war, killed in duels, and indulged in pleasures of flesh, gambling and drinking.
Then began the War of Wrath - The Third Thaumaturgical War. The most horrible conflict in the history, that - for now - surpasses the horrors of even the Great War. For it wasn't until Ludendorff's coup and Lenin's rise that mass murders of civilian population began to be a commonality. In the War of Wrath, they were common from start.
Magic returned to the world. Magicians emerged from their Hidden World, both the Forbidden and the Erithian Catholic Church. That on its own would not be surprising, it was but a repetition of the previous thaumaturgical war. Unfortunately, this time something else came. A dreadful enemy that the Church later expunged from history.
It was the end of the world. The enemy didn't come to conquer - he came to exterminate. He marched forward, murdering and torturing to death every living being he come upon. Villages, cities, and whole countries were wiped from the map, turned to wastelands for no reason but amusement.
Despite the total mobilization of the Erithian Catholic Church, mundane countries of Europe and even many terrified Forbidden, their enemy was winning. It was a long streak of one-sided massacres. The only one the legend mentions is the slaughter of Constantinople, from which merely few managed to flee.
Human character shows best in the most extreme situations. Many fall into despair... but some shine brightly. Leo fought in the war, becoming known for his near-suicidal courage. The war changed him, hammered the impurities of character out of him. He was discovered to possess an inborn talent for magic, and began as one of many war sorcerers. He became a hero; a symbol for those still fighting rather than waiting for the inevitable. Many gathered around him when the war was encroaching upon central Italy.
His last mission brought him to Monte Cassino. A beating heart of Christian monasticism, one of the most important places of the Catholic Church, was going to be destroyed. An enemy army marched towards it, wiping out all life before them and forcing thousands of people to flee towards Rome, the last major holdout of Men on the Italian Peninsula.
The mission of Leo's army was to rescue as many writings (especially the monastery's magical tomes) and relics from the monastery as possible. However, seeing that the enemy marched much faster than the fleeing refugees, he decided to send the relics and books together with the refugees and make a desperate last stand in the monastery, in hopes of delaying the enemy as long as possible.
The battle survived the extensive censoring mostly untouched, as the mysterious writer wasn't there to witness it. Most of its description was taken from the Iliad. At the time doing so was not plagiarism - it was proving your knowledge of classic texts, and simultaneously showing that the character whose life you described wasn't worse than great heroes of antiquity.
Leo and his army were slaughtered to the last man, but it delayed the enemy army for a single day. This was the turning point in the war.
All of them volunteered. All of them took arms to defend the innocent and willingly laid their lives - for them and for God. And they did it in a cradle of monasticism. Generations of monks - some saints - lived and prayed there. Thousands more associated this place with pure, unadulterated Good. It slowly charged the place with Holy Magic. When several hundred soldiers willingly died - and many in brutal ways - to save others and defend the holy place, the ensuing sudden growth of the Holy Magic charge finally burst the bubble.
The effect was the biggest explosion of magic in recorded history. All forces of the enemy surrounding the monastery were instantly annihilated. But the wave of power didn't stop there. According to some, it reached every place in the world. Both humans and the enemies felt it. The former as a sudden surge of long forgotten hope, and the latter as fear.
It took over two decades for the war to finally end. When it ended, the only remaining option was to rewrite of history. The memories were so traumatic that without forgetting the war, no human could live. The War was scrubbed from the history - and only a single copy of the Hagiography was saved. According to author, because Leo - now Saint - and his soldiers (named on the last page) simply had to be remembered. No matter what.

Historical Basis

The Hagiography is almost certainly based in actual historical events that are no longer history. The Hishbon Aet allows the Church to - in certain, extreme situations - rewrite the past. Old events are replaced with new ones, creating new history (including memories and material evidence).
It's not perfect. The moment it is activated, all humans enter a momentary state that can be summed up as an epiphany. For a split second they understand all the events intending to be rewritten, and what it will be rewritten to. If a single one of them says no, the process will automatically abort itself, and the knowledge of the attempt will disappear from memories.
Because of that, the Hishbon Aet has been rarely used. The original history no doubt existed, however finding any evidence for that is next to impossible. And if they exist, they are well hidden in the forbidden parts of Vatican archives. Most of them purposefully spared (like the Hagiography). Sometimes a sufficiently determined magician can uncover some additional evidence from places beyond time... but the War of Wrath was wiped out from history in an especially perfectionist way.
We know it happened. We know that the sudden drop in population caused by it was explained by the Engine by using the Black Death Plague (which means that up to 200 million people might have died in it). We know that people were so traumatized by the event's and the dreaded enemy that the entire species wanted to forget it. That's all.
There exists no material evidence, but some immaterial evidence. There is a short list of Erithian Catholic Church saints that are supposed to be tasked with evening the odds between the Church and its enemies (especially Forbidden). Devotion to St. Leo offers power. His Holy Magic covers defense and self-buffs, especially ones that make one be able to fight beyond his body limits. Sainthood of St. Leo - patron saint of last stands and self-sacrifice so that other might live - is unquestioned.

Spread

The legend of St. Leo is known to the Erithian Catholic Church. He is one of its major saints. The mainstream Roman Catholic Church considers him a saint, but to uphold the secrecy of the Hidden World, he isn't venerated (and is unknown to all but the highest echelons of clergy). Traditionally, his last stand happened on 17 September, which is his Feast Day.