Under the leather...

Harry wakes up with a start, groaning in pain as he tries to sit up... Only for Kali to push him back down again as he fixes him with an angry glare.

"What have I told you about channelling Demonic Magic through your body, Master!" she exclaims while keeping his head rested on her furry lap.

"... Not to?" he murmurs, trying to move his right hand but grimacing at the sudden pain erupting from it.

Harry's body and soul were in a unique situation, in that he had access to both regular magical energy as well as demonic. He had two cores, one that was powered by his soul, and one that drew power from his Demonic benefactor.

Since he had a soul the demonic magic would be cleansed before he used it in most circumstances, essentially giving him at least double the reserves. Harry had learned to bypass this however, instead, channelling raw demonic energy. This was both reckless and dangerous, but, could also provide significantly more power than when simply utilising regular magic...

She flicks him in the head in response, clicking her tongue at his stupidity, "It has taken hours for me to draw the demonic taint from your arm, master... The amount you channelled almost completely transformed it, melding demon scales to skin and bone..." she sighs, "At least we know who granted you their power in the first place."

...

"What?" he questions, his mind still somewhat fuzzy after being knocked out.

"The Demon God who granted you magic in that exchange you made... My mother keeps an extensive library of every known Demon God, she forced me to read it to understand who I can antagonize, and who I probably shouldn't..." she pauses and smiles to herself before continuing, "The God who granted you your Demonic Magic is called Salinarinth, the Snake God of Death, Destiny, and for some reason, I can't comprehend... Children..."

...

"How'd you know?" he questions.

She shrugs, "Those scales that formed on your arm, Demonic Magic holds the attributes of its owner... While you technically produce your own via conversion through your soul, it still retains some of its origins... Salinarinth isn't your typical Demon God, most have never even seen him or observed his influence, myself included... But stories of his form are well known, making recognising his scales rather simple..."

Harry remains silent, raising his right arm and beginning to unravel the bandages covering it, wanting to see the scales she described... Only to be thoroughly disappointed when he finds a regular, if a bit sickly-looking, human arm.

Kali shakes his head with mirth, "I told you, I drew the magic out of your arm... Trust me, you don't want to start undergoing demonic transformation at your age... The results are not pretty, as I have told you many times before." she says, glaring daggers.

"...Sorry... I. I just panicked when that creature's magic hit me... What happened to it anyway? Did you kill it?" he tentatively asks, wondering if he'd have to summon another familiar.

Kali clears her throat slightly and gestures her head towards the left of the room. Harry's eyes follow the direction and lock onto the creature curled up in the corner. Harry spots its tongue flicking out from its mouth, and accidentally looks into its eyes again, panic rushing through him as he scrambles to look away...

Fortunately, the snake hadn't attempted to attack him again, either the threat of Kali, or the attraction from the Snake God's magic forcing it to remain passive.

"I-Is it awake?" he quietly asks.

Kali nods, "It is... Would you like to talk to it? If not, I could just kill it and let you summon another, setting up another ritual might be annoying, but." she shrugs, "Forcing a familiar bond isn't really worth it in most cases. Most end with either the familiar or the master killing one another."

Harry shakes his head, "I'll speak to it... Maybe it was just panicking earlier?" he says before slowly standing and approaching the large snake. Kali shadows him closely, ready to kill the snake if it made any moves against Harry.

"H-Hello!" he says slightly too loudly in Parseltongue.

The snake's tongue flicks towards him as its grey eyes focus, "You Sssurvived... Good." he states, confusing Harry slightly.

"Wouldn't you want me to die? You're the one who attacked me." he asks.

"You Ssstole me from my land, Sstopped my hunt. I wasss angry..."

"So that's why you attacked me?"

The snake doesn't make any moves for or against his question, "I wasss foolissh, you are Sstronger than I expected... Forgive me..." it asks, sounding somewhat regretful... Though, Harry was unsure if it was being truthful or not.

The revelation of where he got his Demonic Magic from had probably made the snake more accepting in becoming his familiar... After all, had he not said that it would "increase your growth and slowly make your magic match my own."

He imagined the snake would do anything to gain even a fraction of the power he'd received from the supposed snake god that'd blessed him.

Harry inwardly grins to himself, happy that he'd recognised these telltale signs that he'd been taught about... Whether it was luck or not, he'd be able to extract far more from it than he otherwise would have been able to. "I'm sorry for taking you from your lands... Is there something I could do to make up for it?" he asks, strangely feeling as if the snake was grinning at him.

"Yesss and no... I wissh to atone, make me yourss, your familiar." it requests.

...

Harry puts on an act, giving an unsure expression that Kali smiles at from behind him. "Erm... I forgive you... And if you want to return to your land, I think we'd be able to do that..." he trails off.

"No!" the snake exclaims, but pauses at its rather sudden interjection. "No... I wissh to Ssserve you." it says, apparently decided to set aside its pride.

...

Harry hums, "Well... If you really want to, then maybe I can take you as my familiar... But! I will have to modify the bond slightly." he explains, glancing at Kali for permission. She nods genially, accepting his method of extracting worth from the creature.

"Why?" the snake grouses unsurely.

Harry shrugs, "Because the master-familiar bond is decided by the strength differential between each side... I am stronger than you, and will not gain as much as you will from the bond, so it has to be altered to suit us both."

...

The snake thinks for a couple minutes before finally bobbing its head up and down in assent. "I accept."