When the Cult of Skaro was a first established, Dalek Jast was just like any other Dalek. However, when the Cult of Skaro's original fourth member, Dalek Rabe, was killed, Jast was commissioned to take over from him.
Jast aided the Cult in their many missions, and fought against the Cybermen during the Dalek-Cyberman war. He survived the battle and escaped with the rest of the Cult to 1930s New York.
He joined Dalek Caan in an attack on Hooverville after Dalek Sec became a Human-Dalek. However, the other members of the Cult began to doubt Sec and both Thay and Jast joined Caan's rebellion against Dalek Sec.
Jast aided Caan in founding an army, but after the Doctor meddled with their DNA Jast was killed by the Dalek-Humans after they got angry when Dalek Sec was killed by Thay.