For the Speaking-Ring Demon, see Demon Elementals
The Speaking Demon is the demon that inhabited the Way Station. It lived in the basement of the station.
It frightened Jake, but Roland Deschain confronted the creature. It spoke to Roland and delivered him a warning: "While you travel with the boy, the Man in Black travels with your soul in his pocket."
Because he had been taught that only a corpse can speak prophecy, Roland put his hand through the wall and withdrew the demon's jawbone, knowing that this could be useful when dealing with similar spirits.
Susannah Dean had sex with this creature, which was carrying Roland's sperm, and she became pregnant by it. The demon was given the sperm by the Crimson King, whose minion the Demon Elemental / Oracle / Succubus had obtained it by having sex with Roland in The Gunslinger.