The Three Brothers

Long ago, when the world was still new and none of the mortal races were born yet, three godly brothers argued about which one of them had the better idea for how things should be. They all wanted mankind to resemble themselves, but their personalities were vastly different. The first brother was humble, and created humans to be simple creatures that wander the land to gather and hunt for food, content with just living each day. The second brother was very smart and created his people in kind, gifted with the power of magic and always striving to understand the deeper mysteries of the world.
But the third brother was jealous and wanted his people to be the best possible version that could be, and so he gifted them with all the things he could think of. He stole the secrets of magic from the second brother and gave it to his people, and he gave them unending life, immense beauty, and gave them the knowledge to make tools for farming and bending nature to their will. They laughed at the simple hunter gatherers of the first brother, and used the same powers as the wise people of the second brother in the most irresponsible and reckless ways to suit their needs.
At first their population was even in number to the others, but due to the immortality of the third brother's people, the numbers soon started to change. While their numbers grew steadily, the simple people started dying after being pushed away from the lush forests and fields and into the harsh mountains where the magically gifted had built their homes. This saddened the first brother, and he pleaded with the third to tell his people to allow his people to live in the forests again too, but he was rejected, which in turn angered the second brother. He had already been annoyed at the theft of his people's powers, but now the third brother's arrogance and stubborn refusal to let the nomads who were no threat to his people live amongst them was too much.
The second brother warned the third to control his people, or he would, but again was not taken seriously. And so the second brother told his people to share their gift of magic with the first brother's people, and together they fought back against the people who pushed them out, hunting them and killing them at any chance they could get. The third brother was outraged at this unfair alliance, and attacked the second brother while he wasn't paying attention. The first brother couldn't do anything to help as he watched his brother slowly die, and so he ran away, swearing he would not let this deed go unpunished.
In secret he helped guide both his people, and the ones of his deceased brother to become stronger, and gave the most powerful of them the knowledge of how they could defeat even a god such as himself so that they could go and take away that which gave their enemies their power; his brother.

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This is the story of how mankind was created as the first race to inhabit the world, and shape it to their liking, according to humans at least. Most other races heavily disagree that this story is true, as it either conflicts with their own myth about how they and the world came to be, or simply because they doubt that humans alone were that important.
There are many variants of this story, mostly changing the interpretation of which kind of people these three races are meant to represent.
One common version of this story claims the third brother's people were actually the ancestors of the elves, and many humans cite it as a warning against the arrogant high elves even though there is absolutely no proof this is true. Political and religious authorities usually frown upon this version as its only purpose is to stoke the flames of racism against elves, and most kingdoms actually try to maintain at least a neutral alliance with the cities of the high elves within their borders.
The "official" version of the story declares the third brother's people as an entirely separate kind of humanoid being that has been hunted to extinction and does not exist anymore, and that it is impossible to say what they truly might have looked like then because no records exist.
Another variant of this version however claims that, while acknowledging the third brother's people were a different kind of humanoid than what is known today, they did not in fact go extinct and have instead intermixed with the other people, leading to certain bloodlines of people with incredible talent for magic, incredible beauty or other features considered to be extraordinary.
The only thing all the different interpretations usually agree on at least is that humans had no talent for magic before the second brother created the mages, and that the latter were the ones who taught them how to use magic, and that every person born with a magical potential nowadays is a descendant of these mages.