The Technomancers were an advanced race of mages from the early days of the galaxy who combined magic and technology. Based on the planet Aldriss, the Time Lords allied with them during the Last Great Time War; Shadovar told Jarad that trying to explain his people's technology to him was like the Time Lords trying to explain their technology to microscopic bacteria.
They worshipped and drew their power from the Horned Ones. Part of their technology/magic included using chalk circles to keep prisoners, and being able to prevent messages from leaving their world. They could also cast protection spells to keep their world from detection.
During the Last Great Time War, the Technomancers made an alliance with the Time Lords, using their magic to resurrect dead Time Lords, thus offering eternally renewable soldiers. The War Doctor, believed this to be an abomination, due to it forcing soldiers to fight and die in the horrific Time War over and over again, and began investigating the Technomancers.
He discovered that these reanimated carried pieces of the Horned Ones within them and that the true reason for the Technomancers offer was to bring enough pieces of the Horned Ones into the bodies of Time Lords for them to manifest outside their own universe and rule over all life. The Time Lords were nothing but pawns in their eyes.
The Doctor ultimately, after a warning that they must stop, which was ignored; erased their entire species from history using the temporal Weapon of Mass Destruction: The Annihilator, not only destroying their species but changing the past such that their species never existed at all. In moments all traces of their civilisation vanished and only the memory of them in those who had met them remained, as an echo of a people, who now; never were. (AUDIO: Legion of the Lost)