When Harry Potter decided to leave the Deathly Hallows, Death was intrigued. Honestly, he didn't get out much, and had little to no entertainment, besides his potions and the occasional practical joke. After a few years, when he realized that Harry Potter was not going to provide any entertainment, Death got bored again.
A hundred years later, when Harry Potter was decided whether or not he had any desire to live a few hundred more years, Death gathered all the souls Harry Potter had affected directly, and decided to have some fun.
A quick interview with Fate, and he was allowed to create a divergent timeline, provided he didn't take the position as Death from this world for at least a hundred years. Since Death had no real desire for more work, or more power, as his already killed mortals on accident more than on purpose, he agreed. For Fate, he was given a hundred more years to try to prevent his absolute demise, and for Death, he got much needed entertainment, without having to start a fight with War or another so-called immortal.
So, Death went back to the day Harry Potter was born, hopping to make a good friend, perhaps even an equal, or someone who could actually beat his true power.
The moment Voldemort used the killing curse on little Harry Potter, Death appeared. I suppose he wasn't exactly the Harry Potter who was the master of Death before this incident, was he? After his mother survived the killing curse, That blasted thing the last Death had set up. He reached into the now dead frame of the Dark Lord, taking the small portion of his soul that still remained and squeezed it between his fingers.
On one hand, this was both a terrible person, and a Dark Lord. He was both powerful, and evil, making him wholly justified in eating it, trapping him in torture for the countless eons it would take for his soul to pass on to the next faze, either being reincarnated, or actually moving on to a different world. But, even though the soul was both evil and powerful, Death knew from experience that souls of evil people tasted disgusting. Besides, even in this semi-mortal form, if he unleashed his full power entire miles around him would instantly die.
It was as he contemplated this, that the two dead Potters truly saw him for what he was.
The father, James Potter, was the first to act. "Ohh Death!" he called out, trying to sound intimidating, "Please do not take our son! He has truly become a miracle and he is all we have left." he reached for his wand, as if to show that he could duel, but seemed to forget that it was on his mortal body, which now lied dead on the floor below them.
Death chuckled. That's hilarious! Trying to sound like he had any intention of doing such a thing, Death surrounded himself in Magic power, "He has been hit by the Killing Curse. By ancient contract between your peoples and me, I should take his life." he said, temporarily boosting his body into low divinity, and trying his hardest to keep his voice level, and not bust out laughing.
James Potter deflated, but before he could try anything else, his wife stepped in. "I gave my life my life in steed of his! I beg of you! Take me instead of my son!" she shouted, ethereal tears running down her eyes. Now they're just making me feel bad! Death sighed, keeping up his cool persona. "Very well, that is acceptable."
Clearing his throat, Death stared at them, acting as intimidatingly as he could without being noticed as trying. "For this agreement to take place, I request you do one thing, in exchange." he smiled, a cold, cruel smile. One that promised endless suffering and torture. "You must... do my job for me!" he loudly proclaimed, giving them two binding contracts.
They were loose, compared to most of his peers, only requiring them to dedicate a year at a time, and only do their job a few times, about a hundred, times a year. In return, they gain a powerful item, the ability to affect the physical world, being able to see loved ones grow up and die, and so on. It's not like I'm paticually needed in the power department.
James Potter gaped at the bright smile, while his wife started floating up slightly, a sign that she was losing her grip on reality. "Where do I sign?" the male Potter joked.
...Time Skip...
When Albus Dumbledore and the Order of the Phoenix entered the room, they were expecting to see a dead baby, and a dead mother. Instead, they found a dead mother, and a dead Dark Lord, a very much alive baby, and an even more so alive teenage changing the babies diaper, and fussing over something about babies and poop.
To make a long sequence of thoughts very short, that was not what they were expecting.
Dumbledore raised his wand, the Elder Wand of legend, and loudly demanded who the teenage was, partially to figure out what was happening, and partially to tell his allies to ready their own wands to defend if things turned ugly. "Who are you!" he shouted.
The teenage looked back with wide eyes. "Oh! How good it is to see other humans! Living ones! Would you mind changing this little one's diaper for me? I'm afraid I have no experience with such things." the teen chuckled.
Idly, Albus noticed one of the witches besides him, the ever faithful Minerva, did a small flick of her wand, fixing the young boy's diaper without actually having to touch it, like most, and he would have said all if not for the teen in front of him, wizards. The teen sighed in relief, his black cloak, so thin and pure it seemed to be made of pure darkness, swayed with him, partially covering face with the points from its collar.
"Thank you, I wasn't sure I could have done that without puking myself. Always did have a weak stomach for such things." he chuckled, rubbing the back of his head with a hand that was definitely not clean.
Now that Albus had the teen's attention, he coughed lightly, and asked his question again, hoping for an answer this time. "Who are you?" he asked again. The teen stopped, thinking for a second, before answering. "That would depend on your definition of 'who'. I am called many names, titles, which one do you want to know?" he asked.
Albus raised his wand slightly, "What do people call you?" he clarified, worried that he was facing some insane Death Eater. He wouldn't put it past them. The teen smiled, "Oh! Mortals generally call me Death, though some call me the Fourth Horseman." he smiled.
Albus was very surprised. Either this wizard was insane, which was far more likely, or the creator of his wand was here changing a baby's diaper, and failing. Seeing as he felt... something different about the teen, he decided to be cautious. Chuckling slightly, Dumbledore addressed the teen again, "Do you have a more... normal name?" he asked.
Death, or at least the physical incarnation of the concept of Death, thought for a second. "I believe my mortal name would be Louis Death. Perhaps Louis Horseman or Louis Smith, if I decide not to be obvious I suppose." he shrugged.
Albus Dumbledore was not ignorant, despite his blinded optimism, and thought to actually question the teen's claims. "I'm sorry for being so impolite, but would you happen to have any proof of that claim?" he asked. The teen nodded. "Yes. The wand you hold, it's one of the deathly hallows, you currently posses two of the three, which is remarkable actually." he smiled, apparently ignoring the significant loss of color in Albus's face.
Death smiled, surrounding himself in Divinity, straight from his actual body. "One really, really should not question Death, Albus."
...Time Skip...
Harry considered himself a very awesome child. For one, he had Death itself serving his every need, just because he was awesome enough to possibly do something. Not only that, but he could start a zombie apocalypse any time he wanted to, and even though his parents were dead, he could still see them both whenever he wanted.
He lived wherever he wanted to, he supposed. One of the many benefits of having Death as a guardian was that he could sneak him into anywhere. Not only that, but he somehow had an seemingly endless supply of gold and magic tools. In fact, in order to allow Harry to cast magic before he went to Hogwarts, Death had actually given him both a staff that he could control necromantic energies with, and a powerful wand to cast magic with.
Today was different. Death stood in front of him, a black cloth held in one hand, and a picture of Harry in the other. "It's not like I'm going to die or anything!" Harry joked to his incredibly immature guardian. Death smiled back, "Now now, you shouldn't think about death so much, it'll ruin your social life." he joked.
Harry smiled, before turning to the train, magical of course, that would take him to Hogwarts.
...Time Skip...
"I'm exited to see your house actually." Ron smiled. Harry was actually slightly nervous, as he had no idea how his adopted father would react to his friends. Harry settled for a quick warning, instead of a full-on explanation. "Just be careful, my guardian can be a bit... unpredictable at times." he warned.
Both Ron and Hermione waved off his warnings, truly believing that Dumbledore would never have let him stay with someone dangerous.
Sighing, Harry opened the door to his house, expecting anything really. Honestly, Death was very random, having tea with important people, or random commoners, or Queen Victoria, or not at all. The sight they saw, however, was still not something he expected.
Death was wearing a white apron that had the words 'Bio Hazard' on it, leaning over an oven. Behind him, several of his favorite ghosts and workers sat around a table, talking causally. Death's own personal crush, a past queen from long past, saw them first. She smiled slightly, "Death! Your favorite human has come! It looks like he brought friends!" she shouted happily.
Death looked up, smiling brightly. He calmly walked up to the three, ghosts appearing behind him and tending to the oven. "Hello, dear Harry. Who are your friends?" he asked.
Harry sighed, This is going to be a long night.
...Time Skip...
Harry Potter should be in his seventh year at Hogwarts. He should have been happily learning with his friends and family. Instead, he currently was staring down the Dark Lord and his forces. Voldemort, with an army of Death Eaters, stood in front of him. Wands were drawn, and Harry knew that at least twelve were pointed directly at him.
Behind him, though completely unseen, was an army of spirits, not quite ghosts, but not passed on yet. On his right side, Ron stood, wand drawn. After him, Neville stood, both a wand and the sword of Gryffindor drawn. On his left side, Hermione stood, wand drawn the the Resurrection Stone hung on a necklace around her neck. After her, Dumbledore, saved by Death, ironically, stood, his actual, original wand drawn.
Sadly, he was pretty sure Dumbledore would probably just chant continuous shields around them.
As if on an unseen signal, both sides reared back at once, and launched as much magic power as they dared all at once. As the spells reached halfway across the gap between the armies, a figure crashed between them.
Every spell was suddenly nullified, and the weaker wizards, in both armies, suddenly fell down to their knees, or simply fell down, knocked unconscious by the figure's mere presence. Where the figure landed, a ten yard wide crater was formed instantly, and all the grass in the entire field seemed to die instantly.
After a few seconds, the dust seemed to blow itself away, as no wind was blowing.
The figure was pitch black. He held a crude scythe in one hand, casually leaned on his right shoulder, and wore a pure black cloak so thin and unearthly that is seemed to be made of pure shadows, flowing around him, protecting him.
The figure spoke, his words piercing the winds, and driving deep into the humans existence. "Tom Riddle. You have defied me seven times. It is time for you to die." Several Death Eaters quickly launched the killing curse, decided to simply end their opponent's life. The bright green beams suddenly reversed, however, launching them back at the Death Eaters who dared to try to hurt their opponent.
The Death Eaters died instantly, their own spells instantly ripping their souls out of their body. The figure leaned forward. "I have allowed you seven chances at life, by sacrificing seven others, it is time for you to die." slowly, the figure walked towards the retreating Dark Lord. His more loyal, or perhaps stupid Death Eaters stood in front of the man, only to die instantly as soon as they touched either him or the dark shadowlike stripes floating around him. The scythe in his hand remained unused, seemingly reserved for some bigger, more important task.
Slowly, the figure walked forth, silently wading through the army of Death Eaters, and finally reaching the dark lord, protected by only three of his servants. Bellatrix, out of option, ran up to the man, wand raised slightly, "Back away!" she screamed. The figure did not listen, and simply walked forward. Bellatrix pointed her wand at the figure again, preparing a spell, but one of the shadowlike wisps touched it when she was only half done chanting, turning it to dust. Still, she persisted, trying to hold the dark figure back, only to die like everyone else as soon as her body touched it.
Finally, the figure stood in front of the Dark Lord, and slowly raised his right hand, the one not holding his scythe. The Elder wand was held in front of him, only to instantly snap out of his hand and into the hand of the figure. The Dark Lord snarled, "Give me that back you-"
Suddenly, the top half of Tom Riddle's body lay on the ground, lifeless. It was insane, impossible, and even with such a grievous wound, he should have had more time before his life faded away. But that's not what happened. Instead, a lone figure stood between two armies, and had walked through one just to kill one man, and killed them all with a touch or less, even though they were pure blood wizards.
This thought ran through both armies, and immediately both knew who this figure was. This figure, was Death itself. The Death Eaters, or those who had the audacity to claim such a title, ran away as fast as they could, some merely collapsing where they stood, crying.
The figure slowly, ever so slowly, walked up to the opposing army, the Elder Wand clutched in one hand. Slowly, he walked up to Harry. "To whom doth thy wish this to go to?" he asked slowly. Harry stood, utterly shocked. "What?" he asked. The figure did nothing. "The Elder Wand is yours. Do you wish to take it, or give it to another?" he eventually asked.
Slowly, Harry nodded, thinking for a second. "Why not give it to Ron? I mean, I have the Invisibility Cloak, Hermione had the Ressurection Stone, why not give Ron the Elder Wand?" he asked cheekily. The figure nodded, slowly, before the Elder Wand crumbled to dust in his hand. "What!" Ron shouted, slightly annoyed that the cool ancient artifact that went to him.
The figure explained, emotionless as always. "The Elder Wand is cursed to kill its owner. To rectify that for your lifespan, I have fused with your wand. When you die it shall go down to its original power, however." Ron nodded slowly. The figure nodded, very slowly, though it somehow captured the notice of every living person in the room.
Suddenly, the shadowlike stripes and cloak around the figure disappeared, revealing Harry's adopted father.
"Okay! I know you just got out a giant battle, so who wants to have a victory party at the Potter Estate!" he shouted. "All drinks are on me!"
Hints and Tips
Death can be seen many different ways. One of the reasons I didn't write this story myself was because I see Death as a lazy teen with no sense of responsibility. But when he actually does his job, however, he is very solemn. Partially because he doesn't get out much.
Most people see Death as something to be feared though, and you might want to make him a more active participant in the story though.
You could also use Death as a side character, and have him as a relatively minor character with a different main theme.
Random Sentence: I wanted to draw Death as a far more powerful force, but then it would have killed everyone it the battle.