Cellular metamorphosis

Cellular metamorphosis was a discipline that was taught by the inhabitants of Antos IV which allowed the practitioner to manipulate their body to the extent that they became shapeshifters.

At some point during the mid 23rd century, this practice was taught to Starfleet Captain Garth of Izar in order for him to recover from serious wounds that he had suffered during the performance of his duty. Whilst the use of the technique allowed him to repair his physical injuries, this came at the cost of severe mental disturbances that drove him insane. 

By the 2280's, the Orion Victory League had captured the daughter of United Federation of Planets Ambassador Robert Fox and taught her the cellular metamorphosis technique whereupon she was given the task of assassinating her father. She used this skill to board the USS Enterprise and masquerade herself as various individuals during her assignment.