Decayed Master

The Master encountered the Warp Core, which reduced the Master's form back to that of his previous, corpse-like self.

Biography
The Master learned of a device known as the Warp Core, a sentient powerhouse of mental energy designed as a weapon to safeguard the planet Duchamp 331. The Master tracked the Warp Core to Earth, intending to use it to power his TARDIS. Unprepared for its power and underestimating its outside awareness, he was attacked by the Warp Core, having the body he stole from Tremas stripped from him, reducing him to his previous, decaying form. The Master then followed the Warp Core as it arrived on Duchamp 331. (BFA: Dust Breeding)

The Doctor made a deal with Death whereby the Master would have ten years of peace and sanity, at the end of which the Doctor must kill him. The still-scarred Master had become a physician with no memory of his past, and took the name Doctor John Smith. The Master was taken in by Wilston-Croft, and inherited his house when Wilston-Croft died. During his 10 years as John Smith, the Master had become emotionally involved with a woman named Jaqueline Schaeffer.

At the end of the ten years the Doctor duly arrived but strove to avoid fulfilling his side of the bargain. The Master became aware of the Doctor's role in pledging him to Death as her servant but forgave him for it. Death herself was present then, disguised as the Master's maid. Death manipulated events so that the John Smith persona would crumble and the true Master become dominant once more. (BFA: Master)

Appearance
After being attacked by the Warp Core, the Master wore a mask and gloves to hide his disfigurement. (BFA: Dust Breeding) After becoming John Smith, the Master did not hide his disfigurement and wore a suit instead. (BFA: Master)

Personality
As John Smith, the Master was still somehow deeply aware of his dark nature and became troubled by it. (BFA: Master)

Habits and Quirks
As John Smith, the Master's favourite desert was marinated figs with a raspberry coulis, he grew tomatoes, made his own wine, enjoyed theatre, books, and the company of friends. The Master was not fond of dogs or people with shifty eyes. (BFA: Master)