Pretty much the villain to beat all other villains, the Devil himself is one the Doctor was pretty much defeated by until Rose came to the rescue. Last seen in the Series 2 episodes The Impossible Planet and The Satan Pit, the Beast was a demon imprisoned in the centre of Krop Tor. It claimed to have existed before the creation of the Doctors universe, which was backed up by writing that the TARDIS could not identify. The Beast also identified himself as Satan, among all of the other religious representations of him. His physical body was a massive humanoid with crimson skin, large ram-like horns and a corpse like face, although it was the mind of the creature that proved to be its power. Theres pretty much an endless supply of episodes and themes that could be explored with this character returning. Although his physical body was destroyed, its easy to imagine his mind is still out there somewhere. Perhaps it survived and headed to a new planet, most probably Earth, taking over the minds of the people who occupy it and turning the world into his own personal Hell. Not only would that provide a physical challenge for the Doctor as he would likely try to liberate the planet. The Demon could also challenge him mentally, as weve already seen, perhaps even being the controlling force behind the Dream Lord of the previous post.
