A planet on which all corporeal life died out long ago, leaving the combined minds of its last hundred people to roam space in search of a body.
Zetar is the origin of the strange lights seen in the Original Series episode The Lights of Zetar. Every living thing on the planet perished in a catastrophe in the distant past. The thoughts and will of the last hundred Zetarians survived as a single energy life form, which has wandered for a very long time looking for a host through which it can live again.
The entity sweeps through the library planetoid Memory Alpha, burning out its archives and killing its staff, and then fixes on Lieutenant Mira Romaine, whose mind is unusually open to it.
Once the Zetarians have taken Romaine over, Kirk and Spock put her in a pressure chamber and raise the pressure until the invaders, used to the vacuum of space, are driven out and destroyed.