The Phylosians have evolved from a species of mobile intelligent plant life into a technologically advanced, interstellar civilisation. The Phylosians wish to enforce peace in the Milky Way Galaxy. For this purpose they were building an armada of large Phylosian starships. They have a naturally long lifespan, which is further extended by their greatly advanced medical science.
Some time before the United Federation of Planets was founded, Phylos was visited by Doctor Stavos Keniclius, who brought a strain of staphylococcus bacteria to the ecosystem of the planet. The plague decimated the Phylosian race and destroyed the spore cells of the survivors, leaving them unable to reproduce. Keniclius cloned himself on the planet in order to keep the Phylosians alive and to acquire a specimen for cloning to serve as crew for the Phylosian fleet. When the Phylosians were informed by the crew of the Enterprise that the Federation had already established peace, they were free to focus on the problem of saving the Phylosian species. A clone of Spock remained behind to help Keniclius with this task.
A metropolis on the surface of Phylos is described thus; "The design is futuristic; roadways undulate and spiral, bridges hang suspended by almost invisible materials, angular structures, many of them translucent, blanket the landscape. The city is of gigantic proportions, more than four times the Human equivalent [....] Curiously, the place seems to be abandoned. Weeds, vines and a moss-like vegetation that has already partially covered the various edifices add to the feeling of neglect.
Agmar is a Phylosian who injected Hikaru Sulu with an antitoxin when Sulu was poisoned by the bite of a retlaw plant. He is a follower of Stavos Keniclius 5. Stavos Keniclius 5 is the fourth clone of Dr. Stavos Keniclius, a Human geneticist from Earth's Eugenics Wars era. He is regarded as "master" and "saviour" by the Phylosian people.