Shortly after the Master regained control over his TARDIS, he tried to gain control of a cult so he could harness the power of the Immortals.
He convinced the real cult leader, Hadley, that he could serve the cult loyally, by supplying them with sarg. Unfortunately for the Master, Hadley only intended to keep the Master alive while he was still useful. With no other options, the Master formed a temporary truce with the Third Doctor to stop Hadley's plan. After the crisis was resolved, the Doctor allowed the Master to depart unmolested in the name of their temporary truce. (PROSE: Deadly Reunion)