Poltergeist

Steven and Diane Freeling (Craig T. Nelson and JoBeth Williams) live a quiet life in an Orange County, California planned community called Cuesta Verde, where Steven is a successful real estate developer and Diane, his wife, looks after their children Dana (Dominique Dunne), Robbie (Oliver Robins), and Carol Anne (Heather O'Rourke). Carol Anne awakens one night and begins conversing with the family's television set, which is transmitting static following a sign-off. The following night, while the Freelings sleep, Carol Anne fixates on the television set as it transmits static again. Suddenly, a hand of a white apparition blasts from the television screen and vanishes into the wall, triggering a violent earthquake in the process. As the shaking subsides, Carol Anne announces "They're here".
Bizarre events occur the following day: a drinking glass of milk spontaneously breaks, silverware bends and furniture moves on its own accord. The phenomena seem benign at first, but quickly begin to intensify. That night, a gnarled backyard tree comes alive and grabs Robbie through the bedroom window. While Steven rescues Robbie before the tree swallows him, Carol Anne is sucked through a portal in her closet. The Freelings realize she has been taken when they hear her voice emanating from a television set that is tuned to an empty channel.
A group of parapsychologists from UC Irvine â Dr. Lesh (Beatrice Straight), Ryan (Richard Lawson), and Marty (Martin Casella) â come to the Freeling house to investigate and determine that the Freelings are experiencing a poltergeist intrusion. They discover that the disturbances involve more than just one ghost. Steven also finds out in an exchange with his boss, Lewis Teague (James Karen), that Cuesta Verde is built where a cemetery was once located.
After Dana and Robbie are sent away for their safety, Lesh and Ryan call in Tangina Barrons (Zelda Rubinstein), a spiritual medium. Tangina states that the ghosts inhabiting the house are lingering in a different "sphere of consciousness" and are not at rest. Attracted to Carol Anne's life force, these spirits are distracted from the real "light" that has come for them. Tangina then adds that there is also a demon known as the "Beast", who has Carol Anne under restraint in an effort to restrain the other spirits.
The assembled group discovers that the entrance to the other dimension is through the children's bedroom closet, while the exit is through the living room ceiling. As the group attempts to rescue Carol Anne, Diane passes through the entrance tied by a rope that has been threaded through both portals. Diane manages to retrieve Carol Anne, and they both drop to the floor from the ceiling, unconscious and covered in ectoplasmic residue. As they recover, Tangina proclaims afterward that the house is now "clean".
Shortly thereafter, the Freelings begin the process of moving elsewhere by packing up their belongings. During their last night in the house, Steven leaves for the office in order to quit his job and Dana goes on a date, leaving Diane, Robbie, and Carol Anne alone in the house. The "Beast" then ambushes Diane and the children, aiming for a second kidnapping by attempting to restrain Robbie and Diane. Robbie is attacked by a clown doll in his bedroom, and Diane is attacked by an unseen force that moves her up the wall and over the ceiling in her room. The unseen force drives Diane to the backyard dragging her into the swimming pool. Skeletons surround her as she tries to swim to escape, but she manages to climb out of the pool and make her way back into the house. She rescues the children, and they eventually escape to the outside only to discover coffins and rotting corpses erupting out from the ground in their yard and throughout the neighborhood.
As Steven and Dana return home to the mayhem, Steven confronts Teague after realizing that rather than relocating the cemetery for the development of Cuesta Verde, Teague merely had the headstones moved and the bodies left behind, desecrating the burial grounds. The Freelings flee Cuesta Verde while the house itself implodes into the portal inside the bedroom closet, to the astonishment of onlookers, while Teague laments the consequences of his greed and foolishness. The family checks into a hotel for the night, and Steven places the room's television outside onto the balcony.

Question: Who is the demon known as?
Answer: The Beast.