SCP Foundation
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The SCP Foundation is a writing site focusing on scientific descriptions of terrifying, confusing, or just plain weird monsters, events, locations, etc. Expect plenty of REDACTED all across the website. The community prioritizes quality writing over pretty much everything else, and as such it is THE place to go if you like top-tier fiction writing, as the fucking huge critic community will RIP'N'TEAR bad articles to pieces. (On the flip side, if you want to read some hilariously bad writing, go to the Lowest rated articles page. You will know the true meaning of stupid.) The SCP Foundation is primarily GRIMDARK, but there has been a noticeable trend toward a lighter tone in the community.
For /tg/ purposes, it's got a load of ideas for your horror games from Delta Green to World of Darkness and everything in between.
Origins
Basically, SCP Foundation started when the Creepypasta archive of 4chan eventually was inspected by a bunch of people who imagined a worldwide conspiracy to suppress said creepypasta events like "If you visit the ninth floor of the apartment at midnight a spirit will assrape you and split your head open" kind of shit. Said fictional organization would send agents to contain the existence of supernatural phenomena, and if it can be detained, preferably keep it captive or stored, or destroy if too dangerous.
So it eventually shaped into a fictional wiki where thousands of "SCP"'s, items, phenomena, or persons are contained, not always peacefully. Extensive descriptions and containments procedures are the most favored part, as well as UTTERLY FUCKING HORRIBLE REALISATION that your world is a clusterfuck of horror which out-edges Warhammer 40K in comparison. SCP-682 is an UTTERLY invincible creature that continuously attacks everything in existence, only kept at bay with being doused in corrosive acid because it enjoys things and foods and environments completely deadly and horrible to humans.
And that's just the tip of the iceberg. One SCP is a genetic phenomena that makes the dreaming person's dreams real. That's right. Everything in reality, from World Wars to Empires is rewritten in one night's sleep. The Foundation's obvious response is to capture, detain, drug them into complete and total amnesia, use them once and kill them, and then AUTHORIZE GENOCIDE OF every population that may show the phenomena. Another is Nikolai Tesla's Reverse Entropy Tesla Gun that will DESTROY EXISTENCE in a few decades, with NO WAY OF STOPPING IT. Then there's rock eating fish that slowly eat California's seabed to destroy one half of United States and the foundation failed to kill them. And there is thousands more, and more to come.
Famous SCPs
SCP-173: The Statue (AKA the Original) - A statue that kills you if you don't look at it. Started the whole thing, and yes, Whofags, it came before the Weeping Angels.
SCP-682: The Hard-to-Kill Lizard - A FUCKHUEG lizard that hates everything (except SCP-053, for some reason) and cannot be killed. There is a LONG list of attempts, none of them successful.
SCP-343: God - The (second?) most powerful reality bender in the world.
SCP-914: The Clockworks - A massive clockwork machine that has 5 settings: Rough, Coarse, 1:1, Fine, and Very Fine. Put something in the box, choose your setting, and pray the result isn't too radioactive.
SCP-239: The Witch Child - Similar to 343, but more GRIMDARK because it's a little girl.
SCP-055: [unknown] - The perfect anti-meme, something you can never remember. Wait - is there even a 55?
Drama
When you get a group of people to do something together, drama is inevitable. The SCP community has a number of big-names calling the shots, and what they say goes. They set the tone for what's allowed and what's not, and if you piss off the wrong person for a trivial thing or hurt someone's massive overinflated ego, you can expect your stay on the website to be short. It kind of mirrors TVTropes in that regard: the front end has a lot of nice reading but on the community side of thing's it's a dumpster fire of misery, egos and dicks. We do not prefer to give examples but utterly retarded articles written by a newcomer cute girl may stay a long time and any criticism will result in a ban.
Early forms of SJWism have also started to surface, with articles spotting unusual genders without them being played for horror.