The Valeyard

Title meaning "Learned Court Prosecutor". The Valeyard was the amalgamation of the Doctor's darker side, between his twelfth and thiteenth regenerations. Afraid that the Sixth Doctor would learn the truth about the Ravolox stratagem, the corrupt High Council of the Time Lords brought the Valeyard into being. They offered him the Doctor's remaining regenerations if he would destroy his earlier Incarnation. The Valeyard prosecuted the Doctor at his Trial, introducing false evidence from the Matrix concerning his adventures on Ravolox and Thoros-Beta, including a false account of Peri's death.  He later accused the Doctor of genocide for having destroyed the Vervoids. Thanks to the Master's assistance, the Doctor eventually exposed the Valeyard, who then fled into the Matrix. The Doctor followed him there to the Fantasy Factory, a Dickensian business owned by J.J. Chambers. Hiding behind the identity of Mr. Popplewick, the Valeyard plotted to murder the Time Lords attending the Trial with a Particle Disseminator. He also trapped the Master and Glitz by replacing the stolen secrets of the Matrix, which they sought, with a Limbo Atrophier. The Doctor eventually thwarted the Valeyard's plans, but the evil Time Lord escaped again, taking over the body of the Keeper of the Matrix

RAVOLOX:  In order to obtain the Time Lords' secret of time travel, spies from Andromeda (the "Sleepers") successfully infiltrated the Gallifreyan Matrix, choosing Earth as their base of operations. When they discovered the Andromedans' plan, the Time Lords struck back. The actions they took against Andromeda itself are unknown, but they used a Magnotron to move Earth (and possibly the entire Solar System) by several light years to another part of space, and rechristened it Ravolox. When the Sixth Doctor first set foot on Ravolox, and learned that it was Earth, he claimed that two million years had elapsed; however, historical and archeological evidence make this duration unlikely. This action, which seems to have occurred c. 14,000 AD, resulted in considerable death and destruction, but was blamed on a freak cosmic phenomenon (a fireball). 

The Ravolox Stratagem was uncovered five hundred years later, when the Sixth Doctor and Peri visited Ravolox, and was publicly exposed during the Doctor's Trial. Indeed, the Doctor was put on trial because the corrupt High Council of Gallifrey was afraid that he would find out what they had done. On Ravolox, the Doctor met the tribe of the Free, led by Katryca. He thwarted Sabalom Glitz's efforts to steal the secrets taken from the Matrix by the Sleepers. He also destroyed Drathro, the immortal robot who guarded the Sleepers, and ruled an underground civilization of human slaves known as UK Habitat. 

Afterwards, the Time Lords probably moved the Solar System back to its original location in space, where it was "rediscovered". The destruction caused by the planetary move was still being explained away as the result of a "fireball", or "solar flares", when Earth was reclaimed by the passengers of the space ark Terra Nova, and human colonists from other planets.

THOROS-BETA:  Planet with green skies and pink seas. It was the homeworld of the reptilian Mentors, who controlled the Galatron Mining Corporation and many other interplanetary businesses. They sold laser weapons to Thordon and Krontep, which led the Sixth Doctor to Thoros-Beta to investigate. His mind was affected by Crozier's Cell Discriminator, and he was almost responsible for Peri's death when Crozier tried transferring dying Lord Kiv's mind into her body.

VERVOIDS:  Intelligent, mutant vegetal lifeforms created by Professor Lasky, Bruchner and Doland on Mogar. They were mobile, could talk, were equipped with venomous stings and emitted poisonous marsh gas. The Vervoids were dormant when they were loaded onto the Hyperion III space liner, but were awakenened by an electrical light burst. They tried taking over the ship, but were destroyed by the Sixth Doctor, who used vionesium light bursts to cause them to wither and die. He was charged with genocide by the Valeyard for this action.