By 2366, many Zalkonians began to undergo a process in which their DNA would mutate, causing them to transform into powerful non-corporeal lifeforms. They began experiencing isoelectric bursts and acquiring psionic potential as part of this process. The Zalkonian people were frightened by those undergoing the change and their government began exterminating anyone showing symptoms. Some of those affected fled their homeworld but were ruthlessly pursued by the Zalkonian military, including Commander Sunad.
In 2366, the USS Enterprise-D discovered a crashed Zalkonian escape pod, and rescued one survivor that had suffered critical injuries. According to Dr. Beverly Crusher, the survivor suffered major trauma to his head and chest cavity, massive plasma loss, and severe damage to most of his organs. Dr. Crusher claimed that the survivor's body had amazing recuperative powers and that the damaged tissue had been repairing itself at a phenomenal rate. This survivor exhibited almost complete memory loss and so she named him "John Doe". "John" began to undergo the symptoms of a mutation while on board. This mutation often caused John pain and manifested as a yellow glowing energy in his chest. It also had its benefits. Miles O'Brien dislocated his shoulder and after coming into sickbay, "John" healed him with a single touch, much to his amazement. A similar contact with Geordi La Forge resulted in increased confidence on La Forge's part. "John" was even able to revive Worf after he fell from a raised platform and was fatally injured.
Later, the Enterprise-D was intercepted by a ship commanded by Sunad. Sunad demanded "John"'s return, but Captain Jean-Luc Picard refused to turn "John" over without an explanation as to the nature of his crimes. Sunad attacked the Enterprise-D with a weapon that choked the entire crew of the ship (with the possible exception of Data), but "John" stopped the attack. Now that Sunad posed no danger to him, "John" finished his transformation and departed into space. (TNG: "Transfigurations")
They were called Falacians in the first draft script.
According to an internal reference document, listing all the aliens seen in Star Trek: The Next Generation, which was circulated around the time of the production of TNG Season 5, the Zalkonians were given the description of "Humanoids with distinctive, heavily wrinkle-lapped foreheads and cheeks, with ridge lines forming the top of each ear."