Zombie World is an ally of Otter Island which has not yet been revealed in the stories.
In a distant dimension, a "day of the dead" scenario occurred. The newly dead would rise up and begin killing everyone around them. Those they killed got up and killed others. This continued until everyone on the planet had become undead. Once the living were gone, the zombies had no reason to get excited about anything, and settled into a quiet life. This lasted for about a year, after which the zombies were beginning to realize they were starving. The world supply of meat had been completely used up and nothing else was sufficient for the needs of zombies.
In desperation, the zombies sought out other worlds which might be able to feed them, but at each world they visited, they were either driven out or unable to find food. Then, one morning, the zombies stumbled upon a planet called Orgol which was biologically alive. The planet allowed the zombies to move in and feed on its flesh, being completely unable to do any significant damage.
After their brush with starvation, the zombies learned that it's really not a good idea to kill the living and make zombies out of them.
Because the zombies have a lifespan measured in millenia, they find the thought of true death horrifying. They also see Nexicul's living zombies as an utter abomination. Because of these factors, they have allied themselves with the enemies of Nexicul.