The Drumming

The Drumming was a condition that the Master suffered from since his initiation to the Time Lord Academy At age 8, the Master, then known as Koschei, stood before the Untempered Schism, looking into the Time Vortex. As the Doctor said: "Some would be inspired, some would run away, and some would go mad." The Master was one of those that went mad. This manifested by the constant drumming he heard ever since the event, which appeared to worsen as time went on. (DW: Utopia / The Sound of Drums / Last of the Time Lords) Ironically, the Master would play the drums in the 'Gallifrey Academy Hot Five'. (PDA: Deadly Reunion)

The drumming appeared to be a major factor, along with the Doctor having Death choose the Master as her champion, (BFA: Master) and an eventful encounter with the Second Doctor. (MA: The Dark Path), in making the Master the man who would go onto commit acts which would result being regarded by his people as "one of the most evil, and corrupt beings this Time Lord race has ever produced." (DW: The Five Doctors)

The drumming itself was later revealed to have been implanted retroactively into Koschei's mind by Rassilon as a link to later free the Time Lords from the time-lock imposed upon them in the Last Great Time War. When the Master turned the human race into versions of himself, they all heard the incessant drumming that had driven the Time Lord mad over the years. Soon after, the Master realised he could use the combined mental power of the Master Race to send a signal of the drumming through time and space and help the Time Lords return to the universe. They successfully made contact, and a White-Point Star diamond was sent to Earth, which the Master Race recovered and used as a power source for the Gate to pull Gallifrey out of the Time War. Soon, Rassilon and other Time Lords began to arrive on Earth through the Immortality Gate, though the Master's efforts also caused Gallifrey itself to manifest near Earth, threatening the planet with destruction. After the Doctor destroyed the link, the Master attacked Rassilon to save the Doctor, joining his people to die in the "hell" of the Time War. (DW: The End of Time)