Mastho was one of the last of the Daemons. He manifested on Earth in three occasions (1586, 2004 and 2586), though due to his non-linear timeline, his first contact with humanity took place in 2004, then went on to the second visit in 1586 and then to the final encounter on 2586.
His October 2003 manifestation 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)
Arriving in 1586, Mastho encountered the Sodality for the first time chronologically, learning of their experiments to create time sensitives and time channellers. The Daemon disapproved, saying humanity was part of a Daemon experiment and that the sensitives and channelers would taint it, ordering them to destroy them all lest he destroy Earth.
Hoping to satisfy Mastho, they began culling all the time-sensitives they had created, which drew the attention of time-sensitives Honoré Lechasseur and Emily Blandish. By 2586, Mastho was summoned for a third and final time. He revealed that the point of the culling was to strengthen the sensitives and channelers until it was possible to create a sustainable, replicable hybrid that combined both of their attributes, giving the Daemons an army of time travellers they could use for conquest. The Child of Time was to be their first success.
Due to the intervention of Honoré Lechasseur, Emily Blandish and Dr Smith, Mastho was tricked to self-destruct when he attempted to seize control of the Child, and destroyed the Sodality along with him. (PROSE: Child of Time)