The secret disciples of the Master during his time as Harold Saxon used the contents of the Secret Books of Saxon to prepare his resurrection after he was killed by his wife Lucy Saxon. After a few years, they eventually began their plans which involved taking over the prison facility where Lucy Saxon was held and taking a biometric impression from her lips which was one of the elements needed for the resurrection. In addition to that, the Master's ring and the Potion of Life were gathered together with the final requirement being the life force of the Master's disciples which succeeded in bringing about his return.
However, Lucy Saxon was aware of the fact that the Master would likely return. To counter this, she had her family's contacts create a Potion of Death to counter the Potion of Life. The Potion of Death caused the resurrection to go wrong so that the Master was slowly dying, had a ravenous hunger and could shoot bolts of energy from his hands generate by the Master's own life force. (TV: The End of Time) The Time Lords were later able to cure the Master of this "condition". (TV: The Doctor Falls)