Tremas Master

Dying, the Master merged with the body of Council Tremas of the Traken Union creating a new incarnation of himself. This incarnation caused the regeneration of the fourth incarnation of the Doctor.

Biography
On the planet Traken, the centre of the Traken Union, still in his degenerated form, the Master plotted to take over the Source, the power behind the Union when the Doctor and Adric stopped him. By unknown means, the Master did manage to steal the body of Tremas, the father of Nyssa. In this new form he resembled Tremas, but with subtle differences. (DW: The Keeper of Traken)

The Master then travelled to 1981, killed Tegan Jovanka's aunt Vanessa and a police constable with his Tissue Compression Eliminator and ensnared the TARDIS in a gravity bubble. He deliberately perverted the Block Transfer Computations carried out on the planet Logopolis in order to hold the the whole universe at ransom, with an unstoppable wave of entropy as the instrument of its destruction. He realized that the entropy wave presented such a threat that he agreed to work with the Doctor to stop it and traveled with him to the Pharos Project on Earth to do so. He had not, however, given up the plan. The Doctor fell off the Pharos Project radio telescope. The Doctor regenerated making an opportunity for the Master to escape. (DW: Logopolis)

After the Doctor's regeneration the Master kidnapped Adric and held him in a hadron web, later he revealed that this is a trap to get the Doctor. But his plan fails as Adric managed to get a message to Nyssa. Adric was rescued and the Master got trapped in the illusory city of Castrovalva, which he had created by means of Block Transfer Computations, attacked by enraged citizens with the city itself due to collapse. (DW: Castrovalava)

The Master then to the year 140,000,000 BC and disguised himself as the magician Kalid. He controlled creatures known as Plasmatons and attemped to use the alien Xeraphin as allies and transported the Concorde Speedbird 192 to that time period via a time contour. the Doctor and his companions Tegan and Nyssa where on the plan and ended up having Hillusinations. He then lost his control over the Plasmaton's and Tegan and Nyssa entered the Citadel and he revealed his true form. He revealed that he is trapped in the prehistoric times. The Doctor had to help the Master escape and would ultimitly he would regret it (DW: Time-Flight)

In 1215 England, he disguised himself as the French knight Sir Giles and used the shapechanging Kamelion, created by the Xeraphin, as his servant. Kamelion had the power to change his appearance so he changed it to King John. He planed to make Kamelion behave so badly that there would be no Magna Carter. The Doctor took control over Kamelion and then took him into his Tardis thus foiling his plan. (DW: The King's Demons) When the High Council of the Time Lords discovered that the previous incarnations of the Doctor had been taken into the Death Zone on Gallifrey. They asked the Master and offered him a new cycle of regenerations if he complied. He agreed to do so and was given a copy of the Seal of the High Council by the Castellan. The Doctor's third incarnation did not believe him and sees this a plot by the Master. He made a temporary alliance with the Cybermen and offered himself as a guide to the Dark Tower. After the conclusion of this adventure, Rassilon then sent him back to his own time after he had been captured by the companions. (DW: The Five Doctors)

Next, he allied with the Rani (an old acquaintance known to both himself and the Doctor as a member of the Deca on Galifrey) to Killingworth, a British mining village in early 19th century England, and allied with the Rani against the Doctor and Peri Brown They found out about their plan to enrage local miners. But the Doctor traped him and the Rani inside the Master's Tissue Compressor and they escape. But he and the Rani were trapped inside the Rani's TARDIS, which the Doctor had sabotaged. (DW: The Mark of the Rani)

The Master escaped the Rani's TARDIS and got into the Matrix and watched the Doctors trial and The Valeyard agreed with the Master to make false images of the Doctor so that he would get sent down. The Doctor proved this agianst the Master and once again defeated him. (DW: The Ultimate Foe)

The Master then somehow ended up on the Cheetah World and got control over the Cheetah People and the kitlings. He allowed them to go to Perivale and hunt for humans. He also got infected with the Cheetah Virus and waited for a human with the virus he could bond with and escape the planet. He found a man called Midge and used him to get of the planet. The Doctor and Ace also found him and tried to stop him. He killed Midge after he failed to kill the Doctor and then teleported the Doctor to the Cheetah World, where he remained trapped. (DW: Survival)