Yog-Sothothery

The entirety of the stories and/or fictional facts related to the entity Cthulhu or the Old Ones in general. The Mythos is a sort of fake mythology, created by H.P. Lovecraft when he began cross-referencing fictional books, creatures, characters and events in his horror stories, connecting them all to a variety of ancient, space-faring races. The Mythos was and is used by other authors (a trend started by Lovecraft himself, who traded names, stories and ideas with his friends and contemporaries), this gave the mythos an air of authenticity, especially to the mysterious and elusive Necronomicon (also known as Al Azif), an old tome of dark magic, recounting the story of the Old Ones.

Now, how does this relate to /tg/? Well, its mainly due to the numerous games based around it, most prominently Call of Cthulhu. The mythos also has a habit of showing up in lots of other games, and is the direct inspiration for most "unfathomable evil things from beyond the stars" in the community. It has had a huge influence on role-playing games, and Wizards of the Coast acknowledged its influence in the creation of the Aboleth monsters, as well as releasing an "Elder Evils" sourcebook. The Eldrazi were designed by someone that was a fan of the Cthulhu Mythos.