This is venturing into Fantastic Beasts canon where we see another aspect to Voldemort's pet snake. This is set just after the battle. I have taken some liberties with Nagini's slaying, merging both book and movie verse.

A Monster

Inside the castle, there were tearful reunions of survivors, tears shed over the fallen.

Newt Scamander scrambled over the rubble-filled forecourt, his wand tip aglow. Night had fallen. There were gaping holes in the walls of his old school. It made his heart ache at the destruction of what had been his second home during his schooldays. He should have come sooner, but they needed help bedding the dragons. So many dragons from Welsh Greens to Fireballs and Ironbellies. Charlie Weasley had come through reaching out to his fellow dragon keepers across the magical world. Newt could hear his wife chiding him as his foot slipped and he landed on his bottom.

"Found her yet?" Tina's voice called out of the shadows. He could see a bobbing lumos. She was doing her grid-search of the far end of the rubble-field.

"No, my dearest…" Newt rubbed his back. He would need a pain-reliever soon. Riding on dragons was something best left to much younger wizards.

There was a shocked expletive from Tina.

"What's it?"

"I just found her head." They would find the body nearby.

Most would look at the snake's corpse and think monster. The children only saw Nagini, Voldemort's snake-familiar, that had caused so much pain and death. The old couple saw the shell of someone they had known – Nagini, the poor woman doomed from her birth with a blood curse.

The Scamanders remembered the sorely mistreated Maledictus they first encountered in Paris. To be fair to both, they were more concerned with her Obscurial companion back then. After Paris, they had tried to do right by her. Nagini had been enslaved so young and had passed through a succession of often cruel masters that she had no idea where she came from or if she had any family left. Even her name was given to her by a former owner. Skender liked the sound of it and kept it.

They had to apply for identity papers for her after they arrived in Britain. Skender might or might not have her on his licenses. There had been a kerfuffle in the Ministry when some clerk informed Tina Goldstein that her companion would better be registered as a magical beast. In the end the Ministry agreed to issue a special identity pass for the Maledictus.

It was hard for Nagini, caught up in her grief over losing Credence and navigating a world which did not even recognize her as human. She tagged along with Tina for a bit, but MACUSA refused to approve a visa to allow her to move to New York. Yusuf Kama was quickly dispatched on some unknown business. He could not care for Nagini despite his apparent goodwill. Newt offered her shelter in his suitcase and basement, but it was far too awkward an arrangement for him as a single male. The same applied to Theseus.

Bunty agreed to host Nagini in her flat and teach her how to get about. It might have worked out, were it not for the curse. Nagini suddenly transformed into a vicious snake while preparing dinner. Bunty got her family out before anyone got hurt. The commotion brought Aurors, including Theseus. One got bitten and almost died were it not for Newt's tireless efforts in concocting an antivenom as the poor wizard struggled in St Mungo's. The same antivenom potion would later save the life of a father of seven in the recent war.

Nagini was distraught over the incident and the Wizengamot demanded that she be caged as a dangerous beast should be. Locked up in the Ministry's holding cells, there were at least three other unintended transformations. They sent owls to anyone who might have a way of breaking the curse to no avail. Each time, it seemed that the human part of Nagini was slipping further from their reach.

Tina could still recall the first time Nagini tasted hot cocoa with marshmallows, courtesy of the Hogwarts' elves while the Scamanders discussed matters in private with Dumbledore. How she had stared warily at the frothy mug before following Tina's example sipping the delicious liquid. How she had smiled shyly with uncharacteristic pleasure and delight. A glimpse of the Black Lake itself was a thing of wonder for the young woman after a lifetime of cages and ugly cities. She had whirled barefoot on the grass in a gasp of short-lived joy. Yet Credence was never far from her mind. They would never meet again. Nagini was gone from Britain when Credence returned to his family.

During her short time with Bunty, Nagini learned to sew and helped with the housework. Bunty had three nephews and nieces in her care then, as well as an elderly aunt. Bunty's eldest niece taught her to read and write. Nagini enjoyed working with fabrics, sewing lovely dresses from bits of old cloaks and robes. She enjoyed hot chocolate and fudge sundaes. She helped feed and tend the newly hatched runespoors and occamies in Newt's basement. The little creatures loved her.

They sent owls all the way up to the ICW. They were recommended a magical cloister in Albania specializing in housing Maledicti. Newt had protested after paying the place a visit. The cloister was a closed order of sisters, sworn to renounce the world beyond their walls. It seemed very wrong that they should be walling Nagini up again after a lifetime of imprisonment.

Nagini had insisted. As a snake, she was a danger to all around her. She lived in fear of the next transformation.

If I do not come back to myself, please kill me…

It was a stark choice they had to make. Nagini disappeared into the Albanian cloister. The Mother Superior submitted reports to the ICW and British Ministry once a year about her latest charge. Otherwise, little news of her welfare reached the outside world.

In 1932, it was reported that Nagini now spent more time as a snake than as a woman. When she did return to her human form, she appeared greatly confused. The sisters dared not allow her to leave her cell even for meals. Tina suggested they allow her to sew clothes or some other occupation if they were not allowing her out to mingle with others. The isolation could not be helping. Their owls to Nagini returned with the letters still sealed. The reports ceased shortly after. When the Muggle War ended, Newt managed to sneak back into Albania despite the chaos of Grindelwald's defeat and the Muggle hostilities. He found the cloister buildings in ruins. Sometime during the war, a Muggle bomb had pierced through the magical wards of the cloister and destroyed it. They had mourned the loss of a friend.

Until a new Dark Lord rose, accompanied by a snake familiar.

The snake was a terror to all. Newt had seen the damage her venom inflicted on Arthur Weasley. There had been reports of Nagini being used to kill and devour Voldemort's enemies. A favoured weapon. Even Death Eaters lived in dread of her. This was not the same Nagini who enjoyed scones topped with clotted cream and pored over dress patterns with Bunty. This was not the same woman who had wept for Credence when he fell for Grindelwald's lies and forsook her. She enjoyed simple things, like hot chocolate, ice cream, and the wind blowing off the lake.

Newt dabbed at the corner of his eye with his spotted hanky.

"We lost her a long time ago, Newt… She was beautiful, not only in body, but her soul. Such a waste…"

"You are right, Tina… She was magnificent…" Newt, you never met a monster you could not love… Had there been some part of the old Nagini left within the creature? An unwilling rider in the monster? The thought chilled the old couple.

The snake's eyes were milky in death. Their wand light caught the shimmering of opalescent scales. The mouth was still agape, fangs dripping venom. Tina cautiously pulled on her dragonhide gauntlets to pick up the severed head.

"She is finally at peace now, after all this time."

It had been a boy barely of age who struck the blow that put Nagini out of her misery. Young Longbottom, Augusta's grandson.

Newt and Tina exchanged nods. They would do this for their friend together. Tina placed severed head against the ragged neck. They would need to act quickly before some enterprising spark thought of harvesting the corpse for potion ingredients, most of which are likely illegal under the current laws.

"Incendio…" the old couple lifted their wands and waved them over the dead snake. There was nothing at first, then a glowing criss-cross of cracks spread across the head and body. The corpse started smouldering, before bursting into a steady blaze. Newt cast a Clean Air Charm to protect them from worst of the smoke.

"Rest in peace, Nagini…" Tina tore off her poison and blood-stained gloves gingerly and added them to the crackling blaze. The body was really burning now. The head reduced to a charred skull.

The Scamanders waited a bit longer before they returned to the castle, content that the fire would burn itself out on the stones of the yard without further damage.

Perhaps somewhere on the other side a slender beauty would greet a dark-eyed youth in a realm free of curses. Perhaps they might dance by a sunlit lake or share a sundae in a teashop just like the one in Hogsmeade, little bits of innocent joy denied them in this life. No Dark Lords to mar their days.

Author's Notes:

I am trying to explain the difference between the Nagini of the 1920s and the one serving Voldemort in 1990s. Seriously, Grindelwald did not convince her to join his cause, then 50 years later, the snake is hanging off Voldemort?

I know not everyone is a fan of Fantastic Beasts or the Cursed Child or any of the video games versions of Hogwarts.