The Beast (The Impossible Planet)

The Beast was a Demon imprisoned in the centre of Krop Tor. He claimed to have existed before the creation of the Doctor's Universe. He identified himself as Satan, among many religious representations of him.

Biology
The Beast's physical body resembled a massive humanoid with crimson skin, two large, ram-like horns (one of which was broken half way) projecting from its head and a corpse-like face. (DW: The Satan Pit)

The Beast could manifest himself through electronic media, including computer voices and hologram viewers. Its mind could leave its body and possess a single hive mind of the Ood collectively or a single human individually, though not more than one at one time. It had the power of telepathy and, through a host, telekinesis. Possession of a human subject affected the host's skin, causing strange markings to appear all over their face and body; and vocal cords, allowing them to speak at different pitches simultaneously. Whether a single human or a hive mind, the eyes of those possessed by the beast glowed a fiery red. The Beast could also project fire from his mouth. Its hosts could also physically survive in a vacuum indefinitely. (DW: The Impossible Planet)

It was known to have a son, Abaddon who was trapped in the Cardiff Rift on Earth. (TW: End of Days)
 * Some also called the Beast itself Abaddon. Abaddon had a "nemesis" or "evil twin" known as Pwccm. It is unknown if Pwccm is related to the Beast. (TWN: The Twilight Streets)

Origins
The Beast claimed to have existed before the Universe, a claim the Tenth Doctor found troubling to accept or consider as he believed nothing could have existed before the universe.

It is known that the Beast had been at battle with the Disciples of the Light. The Disciples, who themselves existed before the Universe, bound it in giant chains and confined it to a pit in the centre of Krop Tor, a planet in orbit around K37 Gem 5, a black hole. If it ever tried to escape, the field holding Krop Tor in orbit would collapse and the planet would be sucked into the black hole, taking the Beast with it. Despite this, the Beast claimed that it was the basis of the Devil-figure in all religions. The Doctor believed this to be because the Beast's existence seeped into the subconscious of all sentient beings. Deities the Beast inspired included the Kaled God of War and the God of Evil for Draconia and Dæmos. (DW: The Satan Pit)

Encounter with the Walker Expedition
In the year 43K2.1, the Walker Expedition came to Krop Tor, searching for the source of energy that kept the planet from falling into the black hole. The Beast possessed Toby Zed via the text transcribed into the Krop Tor Notebook. The Beast used Toby's body to go outside, where he was seen by Scooti Manista, the base's maintenance officer. At first it tried to draw Scooti towards it by hypnotising her, but she managed to break free. Angered, the Beast, through Toby, began to use its telekinetic abilities to break the glass in the section Scooti was in. Scooti tried to escape, but the doors had been sealed. Soon the glass was shattered and she was pulled out into the vacuum. After Ida Scott and the Tenth Doctor went down to investigate the bottom of the mine shaft, the Base's Ood were taken over, causing them to attack the other members of the Expedition. (DW: The Impossible Planet)

Upon the Ood's defeat, the Beast planned to escape by keeping its spirit in Toby's body, leaving its physical body behind. The Tenth Doctor managed to destroy the force holding Krop Tor in orbit.

The Beast's spirit was destroyed when Rose Tyler caused Toby's body to be pulled into the black hole, followed shortly by the Beast's original body, which was destroyed along with Krop Tor. However, it claimed that it would never be truly destroyed. (DW: The Satan Pit)

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 using his TARDIS' telepathic circuits using those same circuits to contact the Doctor. As Martha, it asked the Doctor if she could rejoin him in his adventures, but he refused. The 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. He 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. (WC: The Beast Is Back in Town)

Personality
The Beast was shown to have had a narcissistic personality, constantly exulting itself and its achievements and referring to itself by various grandiose epithets. It was also exceedingly arrogant, dismissing all other creatures as its inferiors and believing itself invincible, a belief that could easily be true. (DW: The Impossible Planet / The Satan Pit)

Other References

 * When Drew Blayney, leader of the Brimstone Skulls motorcycle club, was possessed by one of the ritual motorcycles, he drew upon the image of the Beast as one of his childhood fears. (TM: Jetsam)
 * Mr Smith once noted that the Beast was on the Judoon's most-wanted list, but was not as high on it as Androvax. (SJAF: Sarah Jane's Alien Files episode 6)

Behind the scenes

 * The "Number of the Beast", 666, has appeared in multiple episodes, with reported appearances including The Christmas Invasion, The Impossible Planet, Doomsday, Voyage of the Damned, Midnight, The Stolen Earth, Planet of the Dead, The Doctor's Daughter, and TW: They Keep Killing Suzie.