Disciples of Saxon
The Disciples of Saxon was a sinister cult who worshipped Harold Saxon.
The cult resurrected the Master with the Potion of Life (TV: The End of Time) as he had instructed them to do should he be killed. When his funeral fire had been burnt down, one of their number somehow collected the Master's ring. (TV: Last of the Time Lords)
The resurrection went wrong because contacts of Lucy Saxon's family had calculated an opposing potion to the Potion of Life in preparation for the Master's predicted return. Lucy used it and sacrificed herself, blowing up Broadfell Prison. All the disciples and she were killed, although the Master survived. (TV: The End of Time)