"Lightsinger" 
(Based on something I was discussing with Moron in the Dischord last night, thought it funny how I could apply it to Karthus.)
I haven't edited this at least a thousand times so it won't be perfect, but here's a bit of lore:
Deliverance
Instead of falling to despair as a consequence of being surrounded by death in the slums of the Noxian capital, Karthus was imbued from an early age with boundless optimism, and a determination to share the joys of life to all who would hear! At first, his mission began with writing rousing sermons for the clergy of the local church -- a task into which he poured himself completely, with ineffable enthusiasm. However, despite his best efforts, as time went on he would often reflect on the solemn, unmoved expressions that slowly but surely begin to make up the majority among the congregants. It drove him mad to think that his dedicated work would see so little response from its audience, prompting him to delve ever further into the ancient libraries of the capital cathedral on a hunt for inspiration from the more esoteric religious texts held therein.
In his search, Karthus would come across an old tome, with contents that went on to describe in excruciating detail the design of Heaven itself! He read of a Utopian paradise of divine angelic beings of light, basking eternally in the endless splendor of the Creator -- free from the strife and death and famine that so plagued the poor slums of the Noxian Capital, and knew he had finally found what he needed.
Much of the descriptions came in the form of rhyming poetry, which he meticulously went about putting to rhythm and practiced in secret for months with rapturous glee, over what was sure to be the greatest sermon ever delivered!
Once he was certain of his readiness, It was easy enough for Karthus to convince his superiors to allow him to hold a sermon for a day in lieu of his years of dedicated service, but the chronically shy, bookish sermon writer truly dreaded what would come next. Nevertheless, he could not trust anyone else to deliver the hymns he'd worked so hard at writing.
Karthus stood before the congregation of soldiers, ordinary townspeople, and nobles alike, all looking to him expectantly.
"I asked to uhh, take the reigns today, so to speak," he began awkwardly, pausing to clear his throat. "Because I was starting to notice a particularly worrying trend."
As he explained his stance -- that audience involvement during these sermons was too small, and so change was needed -- he was met with a number of groans and grumblings of malcontent among the crowd, which certainly did little to improve his already shaky confidence.
The old priest who was usually in charge of the sermons tapped Karthus on the shoulder, who before then hadn't even known he was standing there.
"I will manage, Father," Karthus reassured him in a whisper, despite himself.
As he slammed the giant tome in his possession upon the pulpit, a silence descended upon the congregation.
"More than anything, it is important that we keep in mind the kind of world God intends for us, if we only let him into our hearts."
Karthus immediately proceeded into the first passage - a mournful song that served to put into perspective the sorry state of the mortal world - prompting no shortage of eye rolls and scoffs among people who'd heard this tired old sentiment a million times before. His vocals were wobbly, as one who lacked proper experience, which only further exacerbated the issue.
However, as the congregation was being lead through the gates of Heaven by the second passage, something in the air had changed.
Their cynical expressions were starting to melt, and in its place there was wonder and awe.
Driven by this response, he went on to the third passage which entailed the heavenly armies, with increasing fervor. The light streaming in through the stained glass windows seemed to grow, taking on a life of its own, until it enveloped the entire congregation in a heavenly glow. Karthus thought it a sign from God compelling him onward with praises of "golden watchers" "blades of justice poised to answer injustice" and "may death free us all," becoming moved to the point of tears, and his at first barely audible voice rising to a deafening roar that was moreso a crazed rant than singing.
"Come with me to that place, God's children!" He cried, beckoning them into his arms. Into the embrace of God.
Unfortunately for all of Runeterra, the final words of Karthus's first sermon would go unheard.
As he looked upon the faces of the congregants, eager to see what new emotions he'd elicited in them this time, instead there was stillness. Some sat with their heads hung, eyes closed or staring wide open into nothing, mouths gaping ajar, slumped over or fallen from their seats...
The old priest had fallen, and now lay motionless on the ground beside him.
All prior trickery with the light had dissipated, whether or not it was derived by divine intervention, but Karthus's initial shock was surely soothed by the Holy Spirit still operating within him.
"I did it," he said, an uneasy smile spreading across his lips. "I have freed them!"
From there, as the short lore would explain, he becomes a missionary that travels through Runeterra spreading his lethal gospel, and there's a huge bounty on his head back in Noxus. He may or may not have plans to visit the Shadow Isles at some point, to try his hand at cleansing them.
Basically, he's doing the same thing as standard Karthus only with a fixation on religion and indeed the concept of 'eternal life' in a spiritual sense, rather than death itself. He thinks he's doing God's work by sending people off to heaven with his sermons, but at the end of the day he's just killing them. The essence of the original Karthus is still preserved in the fact that he sees this as an emancipation of people from their worldly pains. The book is a connection between him and the divine, allowing him to channel a divine energy (I like to think of it as aether) which his skin has turned a noticeable purplish hue from contact with (a spin on the supposed affects of exposure to the substance of Aether said to exist in the Greco-Roman representation of heaven) and his eyes become glazed over (implying his acquired blindness toward humanity, despite him still keeping his reading glasses.)
I can imagine this take easily being linked to Kayle and Morgana in the lore, as they're actual divine beings. Maybe they'll end up fighting for control over him and his powers to give them an extra edge in the war against each other? I can see that being a thing.
Personality-wise, just as it's shown in the lore here, he's normally a shy, timid, ineffectual type, but when he starts chanting from that book he becomes a total zealot-Chad.
As for changes to his abilities, they would now arise from him having gained more control over his divine power:
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Passive - Death Defied
Stays the same, with the added implication of it being the result of a divine miracle (but not a full on miracle Karthus can control on a whim like Kayle's Ult, which begs the question that if God is truly empowering him to do his work then why wouldn't he prevent him from dying in the first place?)
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Q - Sanctify
Stays the same...~
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W - Heavenly Gate, Pearly Gate, Gates of Light better name that suggests the pearly gates of Heaven
Just a visual change, resembling the pearly gates of heaven, of course.
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E - Invocation
Stays the same, just has a visual change and has a choir warbling on a single note (the pitch varies with each cast) while the active toggle persists...~
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R - Requiem
Karthus tries to sing a hymn and it's awful, but the rising, heavenly choir that sounds in the background sounds lovely while it casts (enemies who would die from the damage can hear it too). Visually it's more divine and_ light_ looking.
, she goes back in time when things go wrong (for example. when she is caught using magic)
