This is less of an OC and more of an idea of a direction you could take King Calvus IV.
Calvus, Herald of the Deep
Status: Infected, Stage WTF!? (potentially Stage 4)
Insanity Level: ??? (could be 5, could be 0, could be anything in between, impossible to determine)
Whereabouts: Ravenna Castello, sometimes ventures out to deal with particularly powerful or resilient survivors.
Appearance: For someone so utterly changed by the Infection, Calvus looks disturbingly close to his old self. He is taller than he used to be, but only as much as a Tier 1 Atlantean, and while his skin now possesses an unhealthy purple hue, he still bears human features and has a full head of hair, differentiated from an ordinary human only by sharp claws, webbing between his fingers, and gills on the sides of his chest that are normally hidden beneath the barnacle-and-moss-covered armor and crown he still wears. Unlike most infected, his eyes glow purple rather than white.
Personality: Calvus has come through the infection with the ability to speak, think, and use magic intact. How he did this is unknown, though many note the effect his father’s power had on Akursius’s Keep and theorize that the infection interacted with the fragment of Nero Caesar’s power he inherited in unprecedented ways. What is known is that the man he used to be is most certainly dead, replaced by a detached, cold, remorseless monster who claims to serve ‘the Deep’ as its herald, and who will not hesitate to slaughter or infect any survivors who become resilient or powerful enough to threaten ‘the Deep’s servants’ or ‘its plan for us all’. It’s unclear if he’s insane, if he’s a puppet of something masquerading as the cause of the magical corruption, or if he really is serving some malign intelligence that controls the Atlanteans and the Dark Sea.
Abilities: In addition to his already-formidable existing abilities, Calvus has gained a permanent aura effect that causes extreme mental distress in any non-Atlanteans in his vicinity, rapid regeneration that causes him to heal from all but the most grievous of wounds in the blink of an eye, massively increased (physical) speed and reflexes, the ability to harness the magical corruption of the sea as an effectively limitless source of magical energy, and the ability to telepathically communicate with and control Atlanteans in a massive radius around him. Moreover, he retains all the cunning, skill, and tactical acumen he had as a king, allowing him to coordinate Atlanteans in his vicinity, thus turning a animalistic mob into a frighteningly efficient and deadly army. And his connection to the Dark Sea allows him to flood a victim with magical corruption through physical contact, infecting them and causing them to progress to the Mutated stage in a matter of hours. Between all that and the truly cataclysmic amount of magical power he can throw around, it’s little wonder why every encounter with him so far has ended in the death or infection of literally every single human who had the misfortune of attracting his (or if he’s to be believed, ‘the Deep’s’) ire.