The Horror was an amalgamation of beings which had been abandoned in the Time Vortex. When the Time Lords sent Chris Cwej into the bottle universe, they unintentionally created a gap through which the merged entity invaded the bottle universe's operating system. (PROSE: Dead Romance)
Some of the beings that were part of the Horror were Pool, (PROSE: Deceit) Salamander, (TV: The Enemy of the World) Ruath (PROSE: Goth Opera) and Father Kreiner. (PROSE: Interference - Book Two)