Bors (The Doctor's Meditation)

Bors was an Englishman living in Essex in the 12th century. When the Twelfth Doctor saved him from a splinter, Bors pledged himself to the Time Lord as a debt for the Doctor "saving his life." Unbeknownst to him or the Doctor, he was turned into a Dalek puppet from a snakebite; serving as a spy to capture the Doctor's TARDIS.

Biography
Bors was with the Doctor while he was trying to meditate in a castle. Bors and his men began to dig a well on the Doctor's orders when he claimed the water they had wasn't good enough. It took them nearly a week to find water, and the Doctor suggested they build a visitor's centre once the well was complete, annoying Bors.

Bors was also able to solve the Doctor's magic trick, but admitted he still thought of himself as an idiot. He also noted that the Doctor was unable to meditate due to the fact he couldn't remain silent, but then listened intently as the Doctor told him about a battlefield he had found which would be his last. (TV: The Doctor's Meditation)

Bors later challenged the Doctor to an axe fight, to which he arrived playing an electric guitar and riding a tank. Distracted by the Doctor's presence with the crowd, Bors was strangled by one of Colony Sarff's snakes. The snake bit him and injected him with a venom full of Dalek conversion nanogenes.

After Colony Sarff took the Doctor prisoner on his ship, Bors fully turned into a Dalek puppet. Bors, with his free will lost, located the Doctor's TARDIS for the Daleks. (TV: The Magician's Apprentice)

Behind the Scenes
In DWM 492, a fan asked showrunner Steven Moffat how Bors became a Dalek puppet. Moffat revealed the snake which wrapped around Bors's neck also bit him, and Colony Sarff's venom contained the Dalek conversion nanogenes seen in Asylum of the Daleks.

Bors (The Doctor's Meditation)