The non-corporeal Zetarians
The Zetarians were a race of corporeal beings from the planet Zetar. When their planet began dying sometime around the 1300's, the last 100 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, minds, their very will to live. Now capable of high-warp speeds, this group took to traveling through space for a millenia, seeking a humanoid vessel with a compatible physiology 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 and staff there and burned out large sections of the computer core, leaving a great deal of amassed Federation knowledge lost forever.
After the attack on Memory Alpha, the Zetarians found on board the USS Enterprise a computer specialist named Mira Romaine. Mira appeared to possess the compatible physiology the aliens needed. When they attempted to possess her body, she was able to resist long enough for Mr. Scott and Dr. McCoy to place her within Enterprise's decompression 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")