The Spawnmother was the primary antagonist of the Star Trek: Deep Space Nine novel Unity.
A Neural Parasite, the Spawnmother spent many centuries laying in wait. When the Bajoran leader Shakaar Edon visited the planet as part of a goodwill tour the Spawnmother took control of him and began producing parasites to take over the Bajoran system and finally destroy the Trill.
During this time the Spawnmother learned of previous Spawnmother's failure in taking over the galaxy, as well as other parasite failures. Unable to contact any others of her race, the Spawnmother believed she was the last of her kind.
The Spawnmother began making plans to perform a mass spawning in order to produce a sufficient number of parasites to take complete control of the Bajoran system. Leaving a soldier parasite behind in Shakaar's body, she exited the Bajoran's body so that she could start her mass spawning - which would require her to grow so large in size that a humanoid body could not contain her. A few weeks later what was left of Shakaar was killed by a Trill assassin who had learned that Shakaar had been taken over.
After leaving Shakaar's body the Spawnmother took up residence inside the chamber holding the Orbs of the Prophets. When Captain Kira Nyers found the Spawnmother in the chamber, the Spawnmother was almost ready to begin her mass spawning. Kira opened all the orb containers, exposing the Spawnmother to all of them.
Captain Benjamin Sisko, who had been with the Prophets appeared and told the Spawnmother that she had picked the wrong planet before transporting her to a different realm. In this realm were the Jem'Hadar reinforcements who the Prophets had transported there while they had been crossing the wormhole during the Dominion War.
Seeing the large creature before them, the Jem'Hadar drew their weapons and promptly slaughtered the Spawnmother.
As the Spawnmother had disappeared from the physical universe, the parasites infesting the Bajoran system all released and left the bodies of their victims. Those parasites not killed right away died a short time later.