Humans have many nations and empires in the Old World, and their countries were the first to discover the New World and the Megadungeon. They founded the Great Church, and they "discovered" the race of the halflings. According to them, they are the most natural race, and so are the race that all other races are compared to.
Dwarfs used to control vast swathes of land, both above and below the ground, with their immense empires. In the superstitions of many past human settlements, dwarfs controlled all that was below the ground. However, for some unknown reason, all of the empires of the dwarfs fell, leaving behind great ruins of keeps and castles. Of the dwarfs that existed, most are either dead or insane and delusional. The very few dwarfs that aren't one or the other are adventurers, not because of any grand reason, but because dwarfs really like money, and it is said that there are great treasures in the Megadungeon. Dwarfs are inherently non-magical, being unable to learn the arcane secrets of wizardly magic, but they have innate capabilities that are unknown even to them.
The halfling people were "discovered" by humans living deep within dark and dank jungles, worshiping things labeled as demons by the Great Church and living in huts on stilts. The human nations took in the "primitives" and "uplifted" them, removing them from their tropical homes and forcibly making them join the "modern" nations. According to many, halflings will steal anything not nailed down, and it is a well known fact that they have unnatural agility and reflexes, due to the strange nature of the halflings' muscles and bones. Many halflings speak of more halflings in distant jungles and forests.