Edited: as of November 2021.
Altered the Lore of "Masters of Death".
The Doctor clenched his fist. As he was escorted to the hospital wing.
"I knew I should have brewed it myself." He sneered.
Snape just glared at the dark lord in training. Not liking it when someone acknowledged his failures. However, in this instance, the man was forced to recognize his lack of adequacy to the task...
The Boy dove deeper. "If he had been anything else, he would have died. Rassillion, he DID die. Likely wasted one of his lives before even entering puberty."
That was true enough, the potion got into his lungs, thanks to the Gillyweed. Snape had been told the boy was just supposed to bathe in it. Well that required full submersion, he should have just had the boy hold his breath.
"I was worried," Snape spoke...mostly reasoning with himself. "That with how painful removing the Horcrux would be, the child would not have been able to stop himself from screaming under the potion."
The Doctor growled, "I should have done it myself..."
Once they reached the hospital wing, spying the unconscious little boy in one of the beds... John... froze. He just stared at the child.
Madame Poppy gave a sigh of relief. "Thank goodness you're here. You're the only other Time Tot we have available. We'll need you to help ease him through his regeneration."
Snape then handed the boy his Techno magic wand. The child shook his head, getting his thoughts back together, before taking his beloved tool.
He started scanning the child. As informed, the strain of the potion had killed the boy. It wasn't meant to be used on the living, and a human mind couldn't compensate for everything needed in reworking the potion.
The boy might have died, but by fluke, that was what was needed to activate his third chromosome chain... an unnatural death. Thankfully, for the boy, the chain contained traces of Time Lord DNA.
The Doctor... didn't think there were any of his people left. The Moment had destroyed them all... he had to... but this boy... this meant others could have survived. And just who survived... the Doctor could recognize her magical signature anywhere.
Shaking his head, he focused on the task at hand, saving the little boy. "His DNA is Spliced with Eternal, your alien ancestors."
Snape found it annoying the child assumed they didn't know what an Eternal was. Though he didn't comment.
The Doctor went on, mostly talking to himself. "Has the usual trace of Native Earthling, that comes with being a wizard... I don't get it... Time Lord's and Eternals have incompatible genes. We shouldn't even be able to reproduce with a squib..."
He raised his wand and looked at it. "More limited regeneration energy. That's common in Hybrids. You shouldn't have to worry about him changing faces... although... with the way the eternal magic is cycling, it appears to have overpowered the genetic block, which restricts the number of regenerations. So being forced to regenerate, at such a young age, won't hurt him in the long run."
The med witch sighed with relief. Snape did the same, as John said, "At least the Horcrux has been destroyed. He should be asleep for a few more hours. I have time."
"Time for what?"
The Doctor sighed, rubbing his head. "The Brigadier is going to have my hide... but he should have learned by now."
Snape was getting frustrated, "Learned what?"
John smiled, with a flash of something darker behind his eyes. "I'm too much of a liar to keep my promises."
Snape did not like where this was going. "Give me back your wand."
The Doctor ignored him. Pointing his wand at an empty corner. It unleashed a loud buzzing sound, as the tip lit up blue.
Before Snape could stop it, a strong wind began to swirl around the spot, scattering the various loose parchment.
Then it happened. A blue crate began to fade in and out of existence. Snape was too stunned by the display of unknown magicks.
"I believe I need a chaperon to leave school grounds," He looked at Snape. "How would you like to meet my Granddaughter?"
The Doctor stepped out of his TARDIS, and into a nice looking living room.
Snape wasn't particularly phased by the bigger on the inside crate. Expansion charms were a fairly common enchantment, for the past 5000 years. Originally invented by the "gods" of Ancient Egypt.
The fact it could teleport was nothing. There were even rumors Time Lords could teleport to other planets in their travel Capsules.
Although, usually, a single TARDIS was home to an entire caravan. Not a single boy. So Snape assumed Unit stole it.
Snape followed John out, part of him wondering why he was doing this... While another was just so shocked John actually had family, let alone a grandchild. It seemed a good opportunity to work on the impossible task Dumbledore assigned him, of molding the boy away from the dark path.
He wasn't expecting to see her... It was rumored Time Lords were capable of Time Travel. But like every sane person, Snape didn't believe they could go back more then a few hours. That was the extent of Wizarding Enchantments.
But the how didn't matter, she was walking over to him, she hadn't aged a day... not since he saw her as a corpse.
"Sev..." Lily smiled. "You've gotten so old..." she acknowledged, before slowly attempting to hug him. Unfortunately, Snape broke away.
"No," Snape shook his head. "No Lily you mustn't, I... it's my fault I..."
Lily's smile turned sad. "Oh Sev, don't tell me your still carrying that burden yourself. I don't blame you."
"I KILLED YOU!!!" Sev screamed. "It might not have been my wand that did the deed, but it was my fault all the same!"
"Severus Snape!" Lily spoke with the conviction that only a red head possessed. "It was. Not. Your. Fault! I have known Old Voldie-wart would kill me, long before that prophecy was uttered."
"This is reckless," John spoke up. "Inserting yourself as Lily Evans? Why would you possibly do such a thing?" he begged, though not for an answer.
Lily blinked, looking at the child... then her smile grew warmer. "Why Grandfather, I almost didn't recognize you. Did you get a hair cut?"
Snape was too stunned to process this. Had Lily just called John...?
"This is SERIOUS!" John snapped, with all the concern one would expect from a parent... strange seeing that on a child's face, but he wasn't a typical child. "Not only is this reckless! But this is your last face! Don't Try to deny it. It's written all over your soul!"
"Oh don't give me that," Lily waved off. "I'm cleaning up your mess. Lily Evans was one of the scars of the Time War.
"Don't get me wrong. The Fates appreciate you gathering the survivors together. But you just dropped them off on your 'Refugee Worlds' so carelessly. They had to do some major manipulations to get history back on track. And like Always, you were no help in rebuilding."
"Lily Evans was one of the casualties. She no longer exist, and would never exist. So I volunteered to fit her shoes." She then looked at Snape, "Don't worry, This was long before we met. Long before I met my parents. And before you get mad about me not telling you, I tried, and you didn't believe me!" Now she looked hurt.
Snape honestly had no memory of her telling him any of this... "It was before Hogwarts, wasn't it?"
Lily sighed, "You thought I was playing make believe. Seriously, can't a girl tell her best friend she's the reincarnation of an Alien Princess-"
"Your not a Princess," The Doctor rolled his eyes.
Lily smiled, "Spoilers," she wagged a finger.
The Doctor groaned, "How's it work? You should be dead Susan! I thought I killed you!!!"
Lily finally lost her smile. Looking deep into the Doctor's eyes... it was if a thousand years of gossip were exchanged in those few seconds of staring.
"I've been acting as a sort of guide, to the survivors," She explained. "Gathered them together, salvaged as many TARDISes as I could, and unlike you, I stayed with them to rebuild. After I died, with all my regenerations used up. I was approached by a Death God. They offered me a job."
She looked back at her Sev, "I get reincarnated a lot. Usually As some orphan Time Tot, left on the steps of some orphanage or clan.
"Usually with my third Chromosome chain dormant, until some key event reactivates it. That Way I age like humans do.
"I don't... actually remember everything. It's just a vague impression at first. Memories of key moments of my life, that could easily be mistaken for dreams. Only when I'm an adult, do I remember all of the details that prove it's real...
"And when I became aware of my mission, I was already married to James... and you weren't returning my calls."
Snape looked away. However, he almost immediately felt Lily's hand guiding his face back to her.
"I'm so sorry, Sev," She sighed, "But at least your here now. How's Harry? I trust you aren't giving Petunia too hard a time. You know how she is."
Snape's heart broke... Lily had expected him to check in on the boy... had expected Petunia to treat him like her own...
Before He could say anything, John snapped, "Spoilers!"
Lily glared at him, "Now that's just cruel. Don't you think a mother has a right to ask after her son."
"How is he even your Son!?" The Doctor distracted, with a question he actually wanted answered. Those were the best types of distractions, in his opinion. "Eternal and Time Lord genetics are incompatible."
Lily sighed, walking a few steps away. Her back to them, as she gently ran her fingers down the TARDIS. "I'm not really sure why he chose us. Or even how he was born a Tenza. The Fates must have been working overtime."
"Tenza," The Doctor's mouth went dry. "You were chosen by a Tenza... a Tenza with the body of a Time Lord / Eternal Hybrid... that boy will have enough power to go toe to toe with gods."
"Let's not forget he's supposed to become another Master of Death. Like me," Lily shrugged.
Snape's head snapped up.
Lily turned back to face them. "Shinigami, or 'Gods of Death'. Are born as a Sapient dimension, although there are other types of those. Their particular breed of Diety grow more powerful, through the more souls they collect. Usually, the souls that don't come with them, get reincarnated, or become ghost. Depending on how attached they are to their past life.
"As Shinigami are rare. They tend to hold jurisdiction over several million pocket dimensions, and related realms. To aid them, each have quite the collection of Reapers working for them. Think of them as Cosmic House Elves. Creatures that feed off the god's magic, through a Psychic link, in exchange for gathering souls to them. The more souls the god gains, the more Reapers they can support.
"Masters of Death aren't Reapers, but we do eventually become that, when we choose. Rather we are living mortals. Champions, or Avatars, of our god. We fulfill a role in balancing life and death. We protect benevolent undead, and destroy the more dangerous. We protect the worlds we are assigned. Often getting reincarnated on planets that need us.
"With my usual job involving giving birth to champions that, due to the scars of the time war, are in danger of never being born. I set the stage for them: Spike Sparkle, Son Goku, Miles 'Tails' Prowler, Naruto Uzumaki." The Doctor was making mental notes of his descendants. "... Harry Potter. My job is to bring them into this world... then die tragically young. I like to spend some time as a Spirit, to watch over them, until my next assignment. Hela doesn't seem to mind.
"If a hero dies too early, other 'Masters of Death' merge with the body, kinda getting reincarnated as that person, or someone to help that person. Hell, even our vacations involve possessing a dead body, and effectively reincarnating ourselves as that person. Although there is a lot of paperwork for that."
John shuddered, although the reason for it may be different then most people's. "Paperwork." The boy shuddered, as if bureaucracy was the most terrifying curse in existence.
Lily smiled, "Regardless, we usually have a dormant third chromosome chain. Containing elements of our past lives. It's how the Doctor can recognize me."
Snape looked at Lily, "Isn't there some kind of rule, forbidding you from telling us what the afterlife is like?"
Lily shrugged, "Not really, there are so many religions out there. No one would believe the real thing, even if you told them."
Snape... blinked... near horrified by how accurate that speculation was.
Lily suddenly frowned, "My apologies. We really should be discussing this over a cup of tea. Come," She guided the two towards the kitchen.