Mongrelfolk are descended from humans cursed by a powerful half-elf wizard to find all other races more attractive than fellow humans and made it possible for them to have viable offspring with any other humaniods. They absorbed not only half-elves and half-orcs but outbred and replaced orcs, halflings, and entire clans of elves and dwarves.
Most mongrelfolk have had the ability to appear to be of another race, just not the race of the viewer. A half-elf might think the mongrelfolk was a tallish and comparatively thin dwarf, and an orc might mistake a mongrelfolk for a human. However, at this point, the sometimes haphazard combinations of features is downright repellant to most other races, though they still find each other attractive since the curse was hereditary. The number of mongrelfolk who can pass for other races is dwindling. Thankfully, they already have a large enough and varied enough population that becoming inbred isn’t really a concern, and those who can pass can do so more easily since the other races are no longer as suspicious.
The mongrelfolk have by far the most contact with the Illumians, and rarely see the other races at all except for their envoys or traders who come to Illumian cities. Mongrelfolk tend to think of the Illumians as bossy but also as the ones who provide an incredible level of convenience and entertainment, so they listen to them.
Mongrelfolk are most often commoners, experts, urban rangers, rogues, witches, fighters (especially the Thug variant) and clerics. Nearly all mongrelfolk consider their religion a major part of their identity, even if they only worship on days of rest and holy days, and many are deeply devout.