Soul Catching

Soul Catching was a rite performed by the Time Lords to transfer their mind into another's before dying, after which they would be assimilated into the Matrix.

In the 1970s, the Third Doctor used Soul Catching on a Waro, which was perhaps the first time it was ever used on a non-Time Lord. (PROSE: The Devil Goblins from Neptune) The experience proved to be so unpleasant for the Doctor that he never attempted it again until his eighth incarnation, when he had to make contact with the entity known as the Beast in order to learn what it was after on Earth (PROSE: The Taint).