The Terminex existed at some point in the distant past. Their dreams of cosmic conquest attracted the attention of the infinitely more powerful Celestials who sought to destroy them to save the universe from them. Realizing that they would not be able to stop the Celestials, the Terminex instead plotted revenge. They began to develop a form of life designed to bring the end of the Celestials' dream of a structured, peaceful universe. At the same time, they designed a ship that would be able to escape their planet as the Celestials attacked, so that their vehicle of vengeance would survive them.
As the Celestials attacked, the leaders of the Terminex at last succeeded in creating a culture of microbes, the Termini. In the instants before the last bastion of their civilization was buried beneath mega-tons of rock and rubble, the Terminex sent their precious burden skyward. Focusing on their attack, the Celestials failed to notice the ships escape, and they began their grim task of sowing the planet's soil and atmosphere with deadly poisons, so that nothing would live there ever again.
The craft launched by the Terminex hurtled through space for centuries, before crashing on an unnamed planet. There the microbes multiplied until eventually becoming that which their parent race had intended: Terminus.