Concerned that she would be pursued once the initial shock wore off, Helena sought refuge in the catacombs under the city. While exploring the tunnels, she stumbled upon an ancient cache of Templar arms and armor. With a determination bordering on fanaticism, she trained herself in the use of it. A year passed in solitude, during which she fasted and claims to have been sustained by faith alone. She awoke one morning with her fingertips bloodied and her sword lying nearby. Smelling the acrid scent of smoldering metal, she picked up the sword and found that runes had been carved into its blade. She realized that she could read the Angelic runes - they read “Absolution”. The sword became a focus for her prayers, manifesting radiant flames along its blade. Taking up “Absolution”, she emerged from her tomb. It was time for her to fulfill her purpose: to purge the world of evil.
Let's go back to my Angels that fell and Atlantis speculation.
What if the magic of Atlantis was based on the powers of those original fallen angels, and was usable by their half-human offspring. The masks would have been created through the powers of beings like Apostate, and can be weilded by humans with enough Nephilim blood, or maybe even all humans if they were designed that way.
Fanatic in this version is an attempt by Apostate to create another angel, not a nephilim, but another being like him. Without other angels he would still be using human beings, but trying to make the Angel DNA dominate the human. He thinks Fanatic is another failure, but when that car hits her and she is dying her Angel DNA triggers, giving her Superman-like flashbacks of her origins, and when she wakes up she's lost all feeling and has not way of knowing what has happened, except her dreams of Angels.
So she enters the convent, and studies while she continues to become less and less human. then she shows her wings, and flees to the catacombs.
What we know happened, she lived in the catacombs for a year, found a cache of old armor and weapons, and didn't eat while she was down there. I think when she went down there her DNA changes were at their climax, and were causing her to go a little crazy, she hides out under the city, starving herself and going crazy, becoming less human and more immortal.
She has a crazy dream and wakes up with her sword alterred (by her unconscious self) and her change largely finished re-enters the world as fanatic.
In this version her powers are tied greatly to her faith, or will. Her sword does what it does because she believes it can, and her own power extends through her sword as a conduit of her will. Similarly the aegis is the focal point for her immortality (which wouldn't be 100%)
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The other option is that she ran into a cache of weapons that did have atlantean roots, so the items have actual powe that augment her own.
I like the idea that Apostate's relics are extensions of his own power, how he uses them is pretty awesome, Relics protecting relics, which also happen to be focal points for keeping himself from being taken out.