Kali

Percy who opens his mouth and instead of human teeth has rows upon rows of shark teeth. When he was a toddler they used to shred his tongue and lips and the insides of his cheeks because they were too big for his baby human mouth.

Percy who likes the taste of blood a little too much. Percy, who, when he injures himself, attaches his mouth to the open wound and drinks his own blood because he likes the taste. Percy who had a painful teething as a baby, and gave Sally innumerable little teeth scars all over her body. (Sally, who loves her little inhuman baby, and allowed Percy to nibble on her forearms even though it was painful.)

Percy with eyes that can adjust to the deep depths— pupils that overtake even his sclera, that take in so much light that he is never truly unaware of what is around him. Percy who looks like he is possessed by a demon because his eyes go entirely black, even the whites, when in the dark.

Percy with a shark's sense of smell. A predator of the ocean, even stronger on land. He knows instinctively that he is top of the food chain, and his nose can track anything.

Percy with bioluminescent spots all over him. He glows in the dark, especially in the ocean, and he uses it to attract enemies toward him so he can kill them.

My take on how Hermes was told his son was destined to die after Percy saw the Fates cut the thread in TLT. Greek translations most likely are inaccurate, but I did my best— they're mentioned at the end.

.

“The thread is cut.”

Hermes startles so badly that he nearly breaks out of his human form. His kerukeion bangs on the floor. He whips around, wings fluttering madly, to glare at his half brother, only to pause at the state of him.

Apollo stands with his veil pulled over his face, the shimmering golden fabric that usually only covers his hair now also obscuring his eyes from view. Even so, the green glow that emanates from them is visible through the cloth. Underneath, the lower part of his mouth shifts between human and wolf, gold ichor spilling out from the cuts incurred by having wolf teeth in a human jaw. The glowing golden circlet on his head flashes like a beacon every two seconds, bright enough that even Hermes feels pain from trying to perceive it. Marble-like cracks run all over his human skin, gold sunrays spilling out from them, the edges of his form fraying and ripping themselves apart, pulled back together only to repeat the process. His half brother looks like he is unravelling.

“Άπολλων,” Hermes begins gently, “ἦ ὐγίαισθα?”

“ὀ μίτος τέτμαται,” Apollo says again, and he sounds horrible, like several voices layered on top of each other, speaking in tandem. He sways dangerously, and Hermes steps forward with his arms outstretched, but Apollo is still speaking with razor-sharp wolf teeth in a human jaw, glittering ichor dripping from his shredded lips down to his chin and sliding lower to stain the white fabric of his chiton an ugly gold.

“ὀ σὸς ὔιος,” he says with ichor covered teeth, and Hermes freezes. “ὀ μίτος τέτμαται. ἄ μοῖρα τέλεσται.”

Hermes stares at his half brother, devastation sinking its claws into his being. Apollo frays even more at the edges under his regard, like cloth being pulled apart thread by thread. “οὔκι, Ἄπολλον, οὐ πίθομαί σοι.”

“ό μίτος τέτμαται,” Apollo says again, like a broken record, a green glow overtaking the gold of his circlet. From under the veil, gold tears start sliding down his cheeks, tinged with poisonous green and sickly white. They mix with his ichor when they reach his jaw, forming a hissing concoction that smells like heatstroke and snake scales. Hermes can hear the despair, the apology, the inevitability in Apollo’s words. “ό μίτος τέτμαται, Έρμάων, ό μίτος τέτμαται.”

Hermes feels the trembling start in his core, expanding outward till his entire being is vibrating, the air around them growing agitated in response to him. He slumps forward, and Apollo lunges to catch him, but both of them are too unstable. They collapse under each other's weight, feathers and scales scattering everywhere at the force of their fall. The cracks in Apollo’s skin grow wider, and Hermes knows his own form is fissuring.

He sucks in a breath, and the wail that he lets out seems to shake the very earth with the intensity of his despair.

Under the veil, Apollo closes his eyes, and the tears keep falling.

.

Translations (a dialect of Ancient Greek):