He (she? it? they?) has gone by many different names in many different times, places, and cultures, but for the most part he is just known as "the Scribe".
The Scribe is surely some sort of supernatural being, though none can say for sure if he is an angel, demon, spirit, fae, or even just an extremely powerful human magic-user. Everyone agrees that he is extremely powerful, though: the Scribe, it is said, has the power to change reality on a whim, to reach into the past and rewrite it, to twist the world to suit whatever its desires might be. And the desires of such a powerful being should be of concern--the Scribe could easily make it so that he is and always was King of the Universe--but the Scribe seems content to play the role of the trickster.
The Scribe, you see, changes people. Sometimes he acts as a dispenser of justice, rewarding the good and punishing the bad, but more often he is arbitrary in his choice of targets, selecting whoever happens to interest him or cross his path. He changes people into other people (or, sometimes, things other than people), plucking them out of their lives and depositing them into new ones, simply because it amuses him. But he does not simply change the flesh, no, he warps reality so that the change not only is but has always been -- so that it was never really a change at all. This is reflected in the minds of the affected and those who know them (or knew them, or would know them, or would have known them). They may or may not accept it at first but by the end of the process they believe that things have always been this way, and memories of the old reality surface only as dreams or a passing sense that all is not right. The changes themselves vary a great deal, sometimes being limited to minor personality shifts and sometimes changing everything about the victim's life. Tweaks to physique, age, gender, race, orientation, intelligence, education, career, relationships, wardrobe, and even species are all fair game to the Scribe. Some say that for especially outlandish changes the Scribe will send a victim into the past or future or another world, but who can say?
The Scribe himself is always nondescript, blending into his surroundings so that he cannot be identified, apart from two distinctive features: he has the most striking orange eyes, and he is always holding some sort of writing tool, whether it be a notebook or clipboard or smartphone or tablet. This is where he rewrites reality--hence the name. Some say that the writing implement is the source of all his power, while others say that it has absorbed the Scribe's own natural power, but all agree that it is important. And if a mortal were to get their hands on it? Perhaps they would gain the power of the Scribe and hold reality in their hands as well, or perhaps the power would destroy them, or even reality itself if wielded improperly. The Scribe certainly would not like to find out, and it is said that any mortal who returns his writing tool to him will be granted three wishes. Of course, we all know how tricky wishes are...
Perhaps you and I have already been rewritten by the Scribe. There's really no way to know. The only question is, who will the Scribe rewrite next?