This is somewhat my pet project and has been the subject of many of my various writings over the past... decade or so. Varying in form multiple times I have come up with a simpler idea.
2000 CE. The turn of the millennium saw more than just the move into a difficult to name decade; minor religious cults had preached about the Apocalypse, the Rapture, the Second Coming and various extraordinary events coming to pass but by and large they had been dismissed by the largely secular world. Unfortunately for humankind the cults were not entirely wrong. Strange creatures began to appear from various religious sites, both old and new. Any place with a place in mythology or any attached legend became a spawning ground for this intruders. Varying from impish monsters barely a foot in height to monstrous behemoths the world, understandably, was panic stricken and order broke down.
Then the military and all their advanced weaponry proved to be ineffective against the invading monsters and all hope was lost. The monsters began to increase in number and they seemed to gain strength from the slaying and consuming of humankind, their power spreading and pushing back the civilizations of men away from these formerly sacred sites.
2001 CE. News spread of a young man, aged 34, who had slain one of the weaker, impish monsters. He had managed this feat with the use of a carving knife when the creature had invaded his family home. More cases of the creatures being slain emerged: all by bladed or blunt weapons such as knives, hammers or improvised weaponry. Hope was kindled as the armies of every nation adapted to the threat and equipped their men with effective weapons.
2003 CE. Ground was retaken quickly from the invading monsters, whose sudden appearance was still inexplicable, as successful tactics were developed and the unorganized creatures were easily slain.
2005 CE. Many of the sacred sites had been retaken by humankind by the middle of this year with the monsters enclosed in small areas around those few sites still not under mankind's control. Morale for the 'war' was at an all time high with the end on sight; the unspeakable fear which had gripped the minds of many fading as quickly as it had come. Just as victory seemed assured humanoid creatures poured forth from the spawning grounds alongside more monstrous creatures. Working in groups and seemingly resistant to even the bladed weapons that had proven so effective until then, these creatures brought back the terror the creatures' first appearance had inspired.
2013 CE. Most of the world has fallen out of mankind's control. Either it has been taken by the invading creatures of shadow or else the land became indefensible and was abandoned to become a No-Man's land. The few holdings men still hold are heavily defended by the newly established Guardians; a force made up of those below the age of 18 at the turn of the millennium. For as of yet unknown reasons this generation are able to wound the newly arrived humanoid shadow monsters where any other man beforehand was unable to do so.
With the formation of this force and the treacherous position mankind maintains it was only fitting that the invading monsters also be named. And what an apt name for scourge of humanity, clad in shadow and often monstrous in shape; the Night Terrors.
This is a brief premise I jotted down a while back, there's obviously a fair bit of history between the lines. There will also be a great deal of questions people will wish to ask. Here are a few simple points to help make things clear:
1. People will RP as Guardians or humans who fit the requirement to be in the Guardians (aged below 18 in 2000 CE).
2. Characters must be between the age of 15 and 30.
3. The sacred sites vary from ancient sites such as the Giza Pyramids and Stonehenge to modern Cathedrals. Any place with mythological, legendary, religious or other sacred significance is a candidate for a spawning ground.
4. Weapons effective against these monsters can basically summarized as pre-gunpowder. However melee weapons directly wielded by the user are more effective than a siege weapon.
I will add to this list as people bring up points. There's a great deal of vagueness but this is, after all, just an interest check. I will embellish upon details as required but I am deliberately trying to leave wriggle room for people to be creative within.