Mastho was one of the last of the Daemons. He took over monitorship of Earth after Azal's death. He manifested on Earth in three occasions (1586, 2003 and 2586), though due to his non-linear timeline, his first contact with humanity took place in 2003, then went on to the second visit in 1586 and then to the final encounter in 2586. (PROSE: Child of Time)
His first manifestation in October 2003 began when a time-active religious order called the Sodality, who had had been following his will since 1586, travelled back in time to revive him, hoping they could control him. Mastho attempted to use Douglas Cavendish to instigate his manifestation, but with the help of Kate Stewart, Cavendish overcame his control and nearly defeated him, but not before the Daemon had escaped back through time with the Sodality. (HOMEVID: Dæmos Rising)
The second summoning was in 1586, where the Sodality wished to harness his powers for their own benefit. Sensing that a Child of Time had been created by humans in this period, Mastho told the Grand Master and his Chancellor to eliminate all time channellers and time sensitives in order to lure it out into the open. He said that once this was done for them to summon him once more in 2586.
At the site of the final summoning, Mastho called Maria, the Child of Time, to him. Maria refused to obey him and escaped into the time vortex, before committing suicide in 1949, the only way she knew of to stop Mastho's plans. Failing in his duty, Mastho self-destructed, consumed by the psionic energies he once commanded. (PROSE: Child of Time)