Death is a constant thing. An irreversible truth. An inevitable fate that awaits every living thing. And maybe because of Death's seeming omnipotence, he could, perhaps, be forgiven his arrogance. You see, Death had been so prevalent throughout the years (thousands and thousands of them) that it had become an entity. It was evolution at its finest; forming one of nature's most powerful and unforgiving children.
Death, he had been aware for many millions of years, but he was peripherally aware that he had technically been around for even longer than that…since the beginning, in fact. The only being older than Death was God himself, but then a couple thousand years ago, God decided he was due for a vacation and told Death to handle things until he got back. Of course Death, being the wonderful younger brother that he was, followed god's request faithfully…until the first few centuries went by and no word was sent from the older sibling. Naturally, Death got bored, and decided to let things play out, only deigning to intervene when global destruction was at hand.
Over time, as he had gotten to the point where he didn't need to be present for every single reaping and the souls would simply go where they needed to, he traveled among the humans. They were interesting creatures, Death decided, they possessed an insatiable curiosity that the other species did not. This enabled them to rise to the top of the food chain and build their own societies and cities. Death watched them as they created countries and cultures, learning as many things as they could on the way. He would have been impressed, if it hadn't been for the countless wars and other smaller conflicts the used to get to that point. Death decided that the humans, curious though they may be, would never be able to accomplish anything, they were always fighting over something with little to no harmony to show for it. Of course it was not too long after he had made that observation, that Death was forced to rethink his conclusions.
It was at some point after the fall of Rome when Death first noticed them. There was a small family of three hiding in their small shack of a house, hunched over a single candle. At first, when Death wandered through, he didn't think much of it beyond the brief, contemptuous, simpletons,that flashed through his mind. Then, just as he was about to leave, he felt it; a sliver of power fill the room and center on the candle. When he turned back, it was to the sight of the child, a young boy no older than four, standing with his chubby little hands outstretched over the wax with a look of intense concentration on his face. There was a short flare of the same power before a tiny flame flickered to life on the candlewick. The woman gasped and the father smiled proudly. The child, finished with his display, tiredly dropped his hands and grinned at his accomplishment. Meanwhile the family was oblivious to the stunned immortal entity standing invisibly in the middle of their house.
Death couldn't believe it! The humans had figured out how to use their spirit to manipulate the elements. That had always been one of the biggest things Death had held against the race; their complete ignorance and refusal to use their own inner power. One of the strongest forces in the universe (aside from Death and God himself) was the power of souls. Humans in particular had some of the largest reserves when compared to any other living thing on the planet. The bigger animals came close, but their lack of intelligence made it impossible for them to utilize it, and Death had though the same was true of humans, but apparently they had figured it out. They were becoming self-aware, Death realized with a start.
After that particular wake up call, Death made it a point to watch out for any other spirit-wielding humans, and found that over the next century, the practice had spread out among the people on every continent. Death silently approved that the people were teaching the others and hoped that it would one day become something every human learned. Eventually, though, he had to admit defeat on that point—powerless humans could be cruel and viscous to the things they didn't understand, and had decided to condemn those that practiced the arts. The mundane section of the population had even chosen a name for those that used the gift, "witches," was the most popular, and Death had a feeling that one would probably stick. Of course, being humans, the newly christened witches had to come up for their own name for mundane; and although Death didn't think "muggles" had any particular meaning, he could tell that this was a derogatory term.
It was around this time that Death had pretty much seen all that he could until the humans decided to upgrade their ever evolving technology again. In his boredom, and admittedly extreme arrogance, he chose to play a trick on the self-proclaimed "wizards." In this trick, Death decided to give a couple of them gifts, and though he would ensure that these "gifts" would have power, he would also attach a lovely little curse to each of the items. Something that would haunt the owners until their death. There wasn't much that could go wrong with Death's prank, except for the little "Master of Death" bit that he'd had to make up on the spot, but it wasn't that big of a deal—there was no such thing as a Master of Death. Sure, if some poor bastard did somehow manage to get all three items in his possession, then he'd have an extremely hard time trying to die and Pass On, but that wasn't really Death's problem. Not yet anyway. He'd deal with that headache when it happened, if it ever did. But when Death considered all the nasty curses and traps and compulsions he'd placed on both the items and the men he'd handed them off to, it wasn't exactly likely to happen. In fact, the probability was something like 0000000000000000.1 chance of even getting two of the so called "Deathly Hallows" together in the same place. Very unlikely.
And so it was the Death went on about his existence, happily ignoring the possibility that he could be responsible for the future accidental immortality that would one day befall a certain hero of the wizarding world.