Keeper of Forbidden Knowledge

A subtrope of Knowledge Broker, the Keeper of Forbidden Knowledge is a being, organization, or even an Eldritch Abomination that contains and sometimes trades information considered either dangerous, blasphemous, epically ground breaking, or any combination of the three. Naturally however, this knowledge comes at a price that makes the trope akin to the Deal With the Devil, and generally leads to disaster for any party involved and then some.

Regarding what form the forbidden knowledge takes, a Tome of Eldritch Lore is almost expected to the exception. However, sometimes the knowledge can come in the form of a living creature, memory, or (in a more modern setting) a large selection of files on a super computer. Whatever form the knowledge comes in, it usually (but not always) feels like Sealed Evil in A Can.

The Keeper itself is typically something ancient, menacing, or powerful enough that no one can simply ask for the information it has without paying a price. Sometimes the Keeper is a title passed down by an order however, and this is generally the oldest or most intelligent of the conspiracy's agents.

Finally, the Keeper usually lives in an isolated or hazardous area of the society or region it lives in, a better way to test would be knowledge seekers of their worth and capability to become puppets for the Keeper's whims, sometimes turning said keeper into a Chessmaster.

The general moral of stories that revolve around this trope is usually that somethings are best left not being known, or that with knowledge comes power, and with power that holds no compassion comes the damning corruption most Keepers of Forbidden Knowledge hope to inspire.

See also Things Man Was Not Meant to Know, specifically The Dark Arts.

Comics

 * Boom Studios Fall of Cthulhu story introduces the Harlot, an obscene Eldritch horror who parodies a burlesque madame. She offers incredible and unspeakable knowledge, but in return the student becomes The Harlot is depicted as a rival and opponent of Nyarlathotep.

Literature

 * Harry Potter has the giant spider Aragog, who isn't necessarily evil, but nevertheless tried to feed the protagonists to his millions of children after giving the young wizards a clue or two about the monster in The Chamber of Secrets.
 * Nyarlathotep from H.P. Lovecraft's Cthulhu Mythos is characterized like this in most of his original appearances. Both in The Haunter of the Dark and The Dreams in the Witch-House he is a gatekeeper to secrets beyond human ken, and must be appeased with human sacrifices in order to relinquish some of this knowledge, and in The Dream-Quest of Unknown Kadath he protects the Gods of the Earth from any mortal molestation. In his original appearance in the prose-poem Nyarlathotep he instead grants humanity information in abundance as a figure of science, leading to the entire world Going Mad From The Revelation.
 * The Doctor Who New Adventures novels have the Library of St John the Beheaded, which collects and stores forbidden texts (both in the sense of banned-by-the-mundane-authorities and in the sense of Knowledge For Which The World Is Not Ready). It's definitely got the isolated or hazardous location part of the trope down pat; on Earth, it was hidden away in The City Narrows, and in The Future it will be located on an asteroid. In principle, it's an aversion of the usual some-things-man-should-not-know corollary: the founder believed that all knowledge is useful if handled carefully, and potential researchers are vetted very carefully before being offered access to the collection. (The Doctor, of course, holds the first ticket the Library ever issued.) In practice, though, pretty much every time it's appeared in a story it's because somebody's found a way to use information from the Library to cause trouble.

Multiple media

 * Tren Krom in Bionicle

Tabletop Games

 * In Dungeons and Dragons, the ancient demon named Dagon (a reference to H.P. Lovecraft) lives within the Abyss, but holds a vast array of information due to his age and intellect, to the point he's become the shadow ruler behind the Prince of Demons' (Demogorgon) entire war stratagem and survival.

Video Games

 * The Pillar Of Skulls in Planescape Torment is one.
 * Hermaeus Mora from The Elder Scrolls games.
 * In League of Legends Nasus holds knowledge of the magic of life and death, although he does teach it to people who are judged worthy by his brother Renekton. The number of people who weren't worthy and wanted the power eventually drove Renekton insane and he started slaughtering first them, then everyone.

Webcomics

 * In Sarab, the Hubs are the only places science may be learned and they are very exclusive.

Western Animation

 * Avatar: The Last Airbender brings us Wan She Tan, He Who Knows Ten-Thousand Things, a spirit in the form of a giant owl who seeks to collect all information in the universe. He once had it all in display in a library for all to see, but eventually realised how humans used this information to hurt and kill others, and decided all knowledge should be forbidden.