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The Pariah was a living weapon created on Gallifrey by the Time Lords.
The Pariah was cast out of Gallifreyan society by Rassilon as it had developed a mind of its own and rebelled.
During a final confrontation with Rassilon, its containment sphere crashed on Earth, where it was discovered by Abraham White. The Pariah offered Abraham White power and immortality, Abraham accepted and joined with the Pariah. The duo later created the organisation known as the Threshold.
The Pariah was killed when the Eighth Doctor destroyed the Threshold base on the Moon by destroying a Threshold device known as the Eye of Disharmony. (Wormwood)