Once upon a time Earth knew peace. The world was united under a single government, people lived in comfort and technological advances had eased if not removed most forms of suffering. A military was maintained, but primarily as a just in case precaution to respond in the event of an unlikely alien invasion. An invasion did eventually happen, but from a different and completely unforeseen source. 

No one knew how it happened, but all over the world portals opened and monsters of endless varieties spread out. The army had little success fighting them, they were able to slightly slow the monsters advances, but with too many loses to have a hope of winning. Even nuclear weapons mostly damaged the Earth, while just temporarily slowing the monsters. Humanity would have been doomed, if not for what is remembered simply as 'the ritual'. No one knows exactly what it was anymore. The time before the invasion has largely been forgotten by now, more myth and legend than fact. Even at the time it was a top secret matter, only a few truly understood what was being done and why.

The immediate result of the ritual was that human women gained a power over male monsters. The monsters were filled with lust, and could not bring themselves to kill the women anymore. Instead they mated with the women, more importantly these women bore the monsters' children (always daughters). These human-monster hybrids became the key that allowed humanity to survive. Victory might eventually be possible, but due to sheer numbers it is still many generations away. Humanity had been reduced to a few large cities that resemble modern cities of today as well smaller and more primitive settlements at this point. For now simply surviving and building the hybrid army is the priority. True victory is entrusted to future generations. 

I will be making a few topics that have more detail on the setting and how all of this works. People could play the monster-girl hybrids, humans of either gender, or monsters. Nearly any monster you imagine could possibly exist here and there is similar variety in the hybrids (the only exceptions being creatures that are simply too large or too small to mate with the human or hybrid women).