The Guide has this to say about the Z'g'r'l:
"The Z'g'r'l are a newbie race of creatures in the pangalactic ecosystem. Their evolution began a scant 13 years ago on the surface of an undistinguished neutron star at the edge of the Sombrero Galaxy. Within 17 nanoseconds of their origin, the Z'g'r'l had developed intelligence and society. In another 5 nanoseconds, they had built a grand city and were looking spaceward. Three nanoseconds later, they had worked out how to encapsulate their grand city and propel it out into intergalactic space, which is quite the feat as you might know had you ever tried to escape the insane gravity of even an undistinguished neutron star.
Once in intergalactic space, the Z'g'ri'l city, which was composed entirely of neutrons, as you must know, quickly sucked up all the subatomic particles it needed to convert itself into ordinary condensed matter. Things then slowed down a lot for the Z'g'r'l, and they found themselves with a lot of time on their hands, which of course is figurative, as they had no actual hands. It took the Z'g'r'l society a full three seconds of intense consideration and debate to come up with a plan of what to do with themselves, which was to 'roam the universe and do good'.
In their roaming over the next year or so, they learned that many of the varied creature civilizations they encountered were self-destructive, engaging in decimating wars over matters that are quite trivial and silly once you think about it. So the Z'g'r'l narrowed the focus of their collective purpose from 'doing good' to 'invading war-disrupted planets to provide the factious inhabitants with an external threat, thereby uniting them against the common enemy and making them all get along better with each other'. It was a brilliant decision and the plan worked brilliantly.
Typically, the Z'g'r'l would go to the planet and hide their grand city behind a convenient moon or something and then send a fleet of robot ships to blow up stuff all around the planet. The inhabitants would at first panic and run around screaming and demonstrating the various specific silly behaviors of their race, until some leaders and spunky fighters with popular, uplifting attitudes appeared, often outcasts or minorities favored by the liberal wing of the society. They would promptly manage some small but hope-inspiring defeat of the robot fleet, and rally ... well, you get the picture. In the end, the inhabitants defeat the Z'g'r'l robots decisively and gloriously, despite an immeasurable inadequacy of everything, and they cheer and hug and feel good, and more to the point of it all, they stop killing each other for a while.
All that time, the Z'g'r'l are in their grand city enjoying the entire history of this world from formation of the planet up to the moment of the invasion, all presented as a neat three-part miniseries, interspersed with live-coverage highlights of the on-going invasion. When the show is over, the Z'g'r'l move on to another warring planet to do more good."