The Beast (also known as Satan) was a powerful demon from the last Universe. Though the exact circumstances are unknown, it is known that he had a great battle with the Disciples of the Light and lost. They sealed him in Krop Tor and placed the planet in orbit around the black hole later known as K37J5. If he ever tried to escape, the field holding the planet in orbit would stop and the planet would be sucked into the black hole.
After the death of the last Universe and the beginning of the next he spread his influence throughout the cosmos. The Beast has powerful telepathic abilities, can speak through electronics and render a crude version of his face on television screens. All the religions which have a Devil or Demon are claimed to be based on him.
After being trapped for many billions of years, the Beast was revived from his slumber by a Human expedition who came to Krop Tor to find the power source which kept the planet in a stable orbit. The Beast possessed Toby Zed, the expert in extinct languages and writing systems, through the text on the Notebook and the pottery. His first action was to kill Scooti Manista by breaking one of the windows (the Beast and those he possesses can survive in a Vacuum for at least a few minutes). The Base's Ood were taken over through their basic telepathy and they became the Legion of the Beast. These Ood rampaged through the base, killing all who tried to stop them. The Ood were defeated when Danny Bartok broadcast a telepathic flare which reduced their field to "Basic Zero" that caused them to collapse. This gave time for Zachary, Danny, Rose and Toby to escape. In reality, the Beast planned to escape inside Toby's body, leaving his body behind. Unfortunately for him, he didn't anticipate The Doctor, who deactivated the force holding Krop Tor in orbit.
The Beast's body was destroyed when Krop Tor fell into K37J5 and his spirit was destroyed when Toby was sucked out of the spaceship and was pulled into K37J5. (DW: The Satan Pit)