Untempered Schism

The Untempered Schism was an opening in the space/time continuum, "a gap in the fabric of reality from which can be seen the whole of the Vortex". (DW: The Sound of Drums) The Schism appeared to consist of an opening into the vortex, contained by a circular ring. Before the Schism lay a plaque bearing the Seal of Rassilon.

Use in Time Lord Initiations
It was used by the Time Lords in an initiation ceremony for children entering the Time Lord Academy. The young Gallifreyans subjected to its terrifying effects would react differently - "Some would be inspired, some would run away and some would go mad." The Doctor and the Master agreed the experience was a painful one (DW: The End of Time). It was claimed prolonged exposure to the Schism over billions of years gave Time Lords their ability to regenerate and indeed was the primary factor in their evolution. (DW: A Good Man Goes to War).

Initiation of the Master
According to the Doctor, many thought it was this ritual which first drove the Master insane. As a child, during his initiation, he gazed into the Vortex, and saw the raw power of time and space - "That's when the Master first saw eternity." (DW: The Sound of Drums).

It was at this moment the Master first heard "The drumming... the never-ending drumming" (DW: The End of Time) that would plague him throughout the rest of his life, perhaps driving him mad.

The drumming sound was in fact a signal sent back through time by the Time Lords in the final hours of the time war in an attempt to escape their destruction. Originally, the Doctor thought it a symptom of the Master's insanity, but after the Master established a telepathic link with him, the Doctor discovered it was real. The signal, according to Rassilon, was "the heartbeat of a Time Lord". The Time Lords used the signal to escape, nearly destroying every universe, but were defeated by the Tenth Doctor and the Master, who aided the Doctor when he realised how he had been manipulated.

Initiation of the Doctor
The Doctor suggested his eight-year-old self was one of the ones who ran away--indeed, that he never stopped running. (DW: The Sound of Drums) Given his character, however, it could be said he was inspired, went a little mad and ran away. The Doctor suggested he found his experience painful. (DW: The End of Time)

Behind the scenes

 * The "time tunnel" effect seen in the Untempered Schism, and reflected in the eyes of the young Master, is the same technique used in the opening title sequence of the Doctor Who TV series from Series 1 to the 2009 specials and during certain episodes from 2005 onwards where the TARDIS is shown in flight through the vortex. It was also used at the opening of DW: The Sound of Drums to represent the transit through the space-time continuum with Jack Harkness's vortex manipulator.