Psychograft

The Pyschograft 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 is somewhere inside the brain, but is slowly being compressed to death. According to the Doctor, it is banned on every civilised planet.

Cassandra O'Brien used the pyschograft on Rose Tyler, possessing her body. Whenever Cassandra wanted to possess another being, she came out as pink gas and inhabited the chosen target. Once Cassandra left Rose, Rose's conscious was back to normal.

Cassandra also used it on the Doctor, an infected patient and Chip. (DW: New Earth)