Psychograft

The Psychograft was a device that allowed the user to possess another living being. In this process, the original body of the user disappears. The target's original mind was somewhere inside the brain, but was slowly being compressed to death. According to the Doctor, it was banned on every civilised planet.

In 5,000,000,023, Cassandra O'Brien used the psychograft on Rose Tyler, possessing her body. Whenever Cassandra wanted to possess another being, she came out as pink gas and inhabited the chosen target. Cassandra also used it on the Doctor, an infected patient and Chip. (DW: New Earth)

At some point around the year 5 Billion, the psychograft was still legal on the planet Fraxinos, where it was used by the Body Bank. Following an unexpected possession by an incorporeal entity, the psychografts were banned. (DWA: The Body Bank)