Magnus, also known as the War Chief, Koschei, the Master, Missy, and various other aliases, was a renegade Time Lord. He/she was considered by the Doctor to be his nemesis. Magnus was responsible for several of the Doctor's regenerations.
- First - This incarnation used the title of Magistrate.
- Tenth - This incarnation went by the title of War Chief.
- Eleventh - This incarnation used the name Felix Kriegslieter.
- Twelfth - This incarnation went by the name Koschei.
- Thirteenth -This incarnation went through several stolen bodies in an attempt to circumvent the regeneration limit.
- Fourteenth - The first incarnation of the regeneration cycle granted by the Tzun. It didn't last.
- Fifteenth - Freed from the Eye of Harmony, this incarnation impersonated the Doctor.
- Sixteenth - Resurrected by the Time Lords, this incarnation resembled a child.
- Seventeenth - This incarnation disguised himself as Professor Yana, his disguise so convincing even he forgot his true identity.
- Eighteenth - This incarnation took the alias of Harold Saxon.
- Nineteenth - The Master's first female incarnation. She went by Missy.