Cthylla

Cthylla (The Secret Daughter of the Dreamer) is a monstrous entity who lives beneath Oakmont. Over time the locals have all but forgotten her true nature and are only partially aware of her existence. They believe that she is a benevolent deity and worship her as Kay;  though she is actually the cause of the widespread madness in Oakmont and across the country. She appears as a gigantic winged-octopus, daughter of the Great Old One Cthulhu, and like her father she is constantly sleeping. Another similarity is that, like Cthulhu, her awakening will probably cause the world's end. For unknown reasons (and by unknown forces), she was imprisoned in Cthygonnaar, a gigantic palace under Oakmont. However, while sleeping, she is still able to communicate with the upper world, and uses her mental powers to call those 'elected' to Cthygonnaar so that they may free her. However, according to a cyclical event, Cthygonnaar periodically erupts upon Oakmont, causing a Flood that spread madness, violence, and monstrosities that prevent people from leaving the city until one of the elected (called "The Seed") either re-seals her for several more centuries or releases her, ending the cycle. Before the event of the game, every Seed has kept Cthylla imprisoned, to prevent the destruction of the city and humanity itself.

By player choice, Charles W. Reed, the Seed of this cycle, could free her, keep her imprisoned or leave the city with the Seal of Chtygonnaar. If Reed frees Cthylla, he will apparently be eaten by the creature, although it is uncertain if Reed is simply killed or absorbed into the supernatural being. If Reed keeps Cthylla imprisoned, he will commit suicide like all his predecessors due to his inability to leave Cthygonnaar, yet ends up preventing the destruction of humanity for this cycle. If Reed left Cthygonnaar with the Seal, he will return to Boston, until the influence of Cthylla expands to the mainland due to the fatalistic nature of the Seal, causing a giant Flood and madness to spread in Boston just like what happened in Oakmont.

Trivia

 * Cthylla is based on the fictional entity created by author Brian Lumley as part of the universe for his Titus Crow stories which are heavily based on HP Lovecraft's Cthulhu Mythos.