From the publisher's website:

In Cthulhu Realms, each player tries to reduce their opponents to insanity or have more sanity than their opponents when the deck runs out. Each player starts the game with a small deck comprised of initiates, goons and followers. Five cards from the deck form a row of available cards on the table; with more than two players, place three available cards between each pair of players. Each turn, you start with five cards in hand, playing those cards in order to put locations into play, boost your own sanity or reduce that of others, collect conjuring power to acquire new cards, and remove cards from the game. You can use the power of your locations once each turn. At the end of your turn, discard any cards still in hand and all entities and artifacts that you played. If you lose all of your sanity, you're out of the game. If you're the only one at the table who's still sane, you win. Otherwise, the game ends when the deck runs out and whoever has the most sanity wins.