From the Game's Introduction:
Chthonian is geared mostly for fans of weird fiction, but can easily be modified for other tastes. Central to the game is the concept of the Narrator and Players. The Narrator acts as the author of the Story, describing events and actions taken by secondary characters. Players roleplay the part of Excursionists, the main protagonists of the Story.
When beginning a game of Chthonian it is recommended that the Narrators and the Players are all on the same page. Be sure that everyone knows the Style of play, the themes to be explored throughout, and what their Excursionists are expected to be able and unable to do.
You will notice that there is no Narrator section here. This is because the Narrator needs to know no extra rules outside of Creating an Excursionist and Mechanics. A good Narrator should remember that the Players and the Narrator are creating a Story together, and should encourage suggestions from Players for the Story and allow the Players the full use of their Nodules.
This game is designed to be played with several levels of Style. For fans of action-packed adventures filled with goons that drop like flies from a hero's revolver, the Style is Pulp Adventure. If the Excursionists only slightly better than average at dealing with the supernatural and bizarre, it is called a Weird Tale. The grittiest, and potentially most dangerous Style is a Shocking Discovery, where most Excursionists will wind up dead, mad, or somewhere in between.
Aside from a pencil and paper, you should have a twelve-sided die, a duodecahedron being the weirdest of the regular polyhedrons. Players use tokens, called Nodules, which can be represented by anything from scratches on paper to pennies to Mancala stones.
It is recommended that you have read some weird fiction appropriate to the Style, in order to play your Excursionist or Narrate to the fullest extent. H.P. Lovecraft is the quintessential author of weird fiction, though there are certainly others.