When their planet began dying some time around the 14th century, the last hundred survivors planned on leaving the planet. Just as they were about to leave, however, one last cataclysmic event occurred, destroying their corporeal existence. They continued living as a pattern of lights and non-corporeal energy, containing their thoughts, desires, hopes, and will to live.
Capable of high-warp speeds, this group took to traveling through space for nearly a millennium, seeking a humanoid with a compatible physiology that they could inhabit and live out their lives through.
The Zetarians desired no cohabitation or symbiosis, but rather intended to take over a humanoid body outright, shoving aside the resident consciousness to make room for their own. They claimed they did not wish to kill, but those who resisted their takeover would die. In seeking out humanoids, they killed countless lifeforms by burning out sections of their brains, causing massive internal hemorrhaging. Their energy patterns also disrupted delicate computer systems.
In 2269, Memory Alpha was attacked by a body composed of ten Zetarians, who killed the entire population of researchers, scholars and specialists there and burned out large sections of the library's central brain and memory core, leaving a great deal of amassed Federation knowledge lost forever.
After the attack on Memory Alpha, the Zetarians returned to the USS Enterprise, and the specialist Mira Romaine, which they had tapped in an earlier encounter with the Enterprise. Mira appeared to possess the compatible physiology the Zetarians needed. When they attempted to possess her body, she was able to resist long enough for Scotty and Dr. McCoy to place her within Enterprise's pressure chamber. After adapting to the vacuum of space, the Zetarians were extremely susceptible to the increased pressure environment, and were quickly driven from Mira's body and destroyed. (TOS: "The Lights of Zetar")