[hider=History of the Demon Races]
Long ago some of the members of the human race at the time, what we call Neanderthals, evolved the ability to project their energy out of their bodies and manipulate it.  This "magic" attracted the attention of a malevolent race of energy-based beings which we would call Demons.  They found that they could inhabit the bodies of the "magical" members of this race by feeding on their energy.

The hosts, however, were unable to survive this condition for long, so the energy based lifeforms began selectively breeding their hosts to create superior hosts.  They even used their host's abilities to alter the host's DNA to make it more compatible and to give it other traits they found superior.  This lead to the rise of several parent demonic races.

There are four known precursor races for demons, with others theorized.  One of these races was larger and more physically imposing than humans, and became the precursor to ogres, trolls, and the like.  One could manipulate energies such as heat and electricity, becoming the precursor to the Ifrit. One had psychic powers, and became the precursor to the imps. Finally, one had the ability to manipulate and feed on emotion and became the precursor for Fear demons as well as the Cambians (Incubi and Succubi).
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[hider=The Rise of the Fae]
Humanity continued to evolve past the Demonic Era and soon a new race had naturally evolved, Homo Sapiens.  A larger percentage of them had the ability to use "magic" than among the Neanderthals, leading to them pushing ahead in military strength.  Even more importantly, they had begun to develop mutations that, while limiting the range of their magical ability, focused it enough to give them specific abilities instead of the more general abilities they gained through spellcraft.

The use of specific magical abilities attracted another form of life, a symbiotic bacteria which fed on magical energy, that had a rudimentary intelligence when it reached large enough numbers.  This lifeform came to inhabit the more powerful of Homo Sapiens and influenced them to form their own communities.  These communities became the early members of a few races, most notably the Elves and the Dwarves.
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We work for the Bureau of Paranormal Investigations, a division of the US government which provides police services in situations where non-humans and magic is involved.  Not all of us are human, but we work to keep the people safe.

There is also a Purity movement which believes that humans should not breed with non-humans, and only accepts pure humans, those whose entire genetic code evolved without influence from Demons, Fae bacteria, aliens, or any other source.  They have tests which will reveal any DNA which didn't come from humans or Neaderthals.  Because only 10% or less of the population is pure human, they will deal will other humans, people who have the majority of their DNA from such sources and can pass as pure human at a glance.

To be continued...