Colony Sarff

Colony Sarff was a servant loyal to Davros comprised of one large and many small snake-like creatures. By working in unison, the snakes were able to mimic a single humanoid individual. However, each snake could act independently, but the colony made decisions as a democracy.

Sarff searched for the Twelfth Doctor, travelling to the Maldovarium, the Shadow Proclamation, and Karn. They encountered the Sisterhood of Karn, who could stop them from using their powers against them. Whilst unknowingly in the presence of the Doctor, they told Ohila, the Sisterhood's High Priestess, that Davros was searching for him.

Sarff returned to Davros, who told them that in order to find the Time Lord they must first find his friends. They then followed Clara Oswald and to Essex in 1138, where one of the snakes attacked Bors. They transferred the Doctor, Clara, and Missy (with their hands bound by more snakes) to their spaceship where they travelled to Skaro. They then accompanied the Doctor to see Davros, who asked them to leave. (TV: The Magician's Apprentice)

Behind the scenes
In Welsh "Sarff" means "serpent".