The Beast (Doctor Who)

"You will die. And I will live."

- The Beast talking through the Ood

"I am the Rage, and the Bile, and the Ferocity! I am the Prince, and the Fool, and the Agony! I am the Sin, and the Fire, and the Darkness! I shall never die! The thought of me is forever: in the bleeding hearts of men, in their vanity, and obsession, and lust! Nothing shall ever destroy me! NOTHING!"

- The Beast through Toby Zed

The Beast is a gigantic demon that appears in the British sci-fi series, Doctor Who, appearing in the episodes "The Impossible Planet" and "The Satan Pit".

Origin
The origins of the Beast are a mystery, but it claims to have been born before the universe began and is the basis of every religion's concept of a Devil. What is known is that the Beast was captured by the Disciples of the Light and chained inside a pit deep within the planet Krop Tor, which was able to orbit a black hole without being sucked in.

Encounter with the Doctor
When a Torchwood exploration group arrive to Krop Tor to discover what was preventing the planet from being pulled into the black hole, with the Tenth incarnation of the Doctor and his companion Rose Tyler appearing, the Beast saw it's chance at freedom and possess the mind of Toby Zed and then the Ood to serve as it's Legion while making it seem that he left Zed's body. Managing to calm everyone down when the Beast preys on their insecurities, the Doctor has Ida rig up the loose cable so he can explore the pit to find the Beast while the others manage to escape with Rose realizing that something is amiss.

Finding the Beast after detaching the cable to reach the bottom, the Doctor learns that Beast had transfered it's mind into Toby's body to escape and that was chained was a demonic body that acts on base instinct. The Doctor realizes that the Disciples of the Light arranged for oxygen within the Beast's prison so someone can destroy the source of Krop Tor's artificial gravity to thwart the Beast's escape as it's mind and body are still linked. As the Doctor destroys the gravity with the Beast's body screaming with Krop Tor is sent falling into the black hole, the Beast's mind with Toby violently reacts to the turn of events before Rose sends the demonic vessel out into space to be sucked into the event horizon, However, it claimed that the beast's original body and sprit would never be truly destroyed.

The return of the Beast
Far from being killed after passing through the event horizon of the black hole, the Beast found itself reconstituted and revived on the other side of the cosmos. It went to Earth where it assumed the form of Martha Jones and used the TARDIS' telepathic circuits to contact the Doctor. As Martha, it asked the Doctor if she could rejoin him in his adventures, but he refused. They were then chased to the TARDIS by the Cybermen, which were actually a psychic projection created by the Beast. There, the Beast revealed itself and told the Doctor that he would use his TARDIS to conquer time and space. The Doctor then asked the Beast to turn into Martha again, so it was small enough to push into the time vortex. The Doctor revealed that he knew all the time it wasn't her, because she would phone him as she didn't know how to use the TARDIS' telepathic circuits.

Trivia

 * The Beast is the father of Abaddon, the main antagonist of the first season of Torchwood.
 * The Beast is voiced by Gabriel Woolf, who also voiced Sutekh in the Classic seaons of Doctor Who.