*~*Demons: The Mythology*~*
In the Buffyverse, the world is pretty much run by humans, with the things that go bump in the night forced to live in sewers, crypts, or...abandon frat houses. According to Joss Whedon, this wasn't always the case. Before mammals and humans evolved, demons, in their pure state, roamed the earth. With the appearance of man the demons were pushed out, leaving them with two options: Flee to other dimensions, or stay.
Those who left are the pure blood demons. They are usually stronger and bigger then your average big bad. Also, they tend to look down on mixed breeds. These demons are rarely seen on BtVS because of the barrier between dimensions. Some do manage to slip through on occasion though. (The Mayor, Glory, The Hellmouth Spawn, and The First.)
The demons that choose to stay behind became more humanlike, and are known as lesser demons. (This doesnt mean they are any less ugly) These demons have had to conform to survive among humans. Some abandon their own languages, some work alongside humans, and others mixed with human blood. Vampires are all human demon hybrids, and are considered the lowest of the low in demon hierarchy.

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*~*Slayers: The Mythology*~*
The Slayer line basically started soon after the arrival of mankind. (Considering the first Slayer was a cave girl with no language and a horrible fashion sense.) The Powers That Be looked down from their clouds and decided they didnt enjoy seeing the helpless humans being attacked by monsters. So, to even the score they chose a girl one girl in all the world who oh hell, you know the deal. (One girl...countless demons...am I the only one seeing a problem in the PTBs counting skills?) When the Slayer dies her power is passed to another, and another. An endless cycle: Girl called, girl fights, girl dies. Needless to say, being a Slayer is not a career where you'd ever see retirement. (Plus the benefits suck) They fight the good fight till they meet their match. (Or, if youre Buffy Summers, you meet your match and come back to life a few times.)
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