The undead is a phenomena that began on a formerly flourishing planet. It began when a rogue scientific starship crash onto the planet, and incidentally released a genetically modified parasite into the environment. Before an environmental protection group was able to get there, the parasite had already infested the whole planet, and had become too numerous to eradicate. The parasite first infected fish, and marine life. The results of the infectious parasite were numerous things; first, the heart of whatever organism is infected, stops beating. This begins a chain reaction that turns an organism into a walking corpse. After blood stops pumping, the parasite moves through stale veins to the brain, and begins eating it away over a few weeks. While this happens, the organism effectively dies, and after that, it ceases any regeneration. Once the parasites have devoured the entire brain, they attach their own DNA to the DNA of the host, and begin creating a second brain. Once this second brain is created, the parasites keep it technically animate, but the organism will remain organically dead, and continue decomposing. As the body decomposes, the parasite switches from using muscle tissue to using radiation in order to illicit movement. The oldest parasites on the host travel to joints and surround bones before dying spontaneously and create a spongy layer to keep the organisms skeleton intact. This process is repeated throughout the newly created organism's life. The end result of this is that after the original organism's corpse is entirely decomposed, the only thing that remains are essentially walking skeletons of the former organism.
Using the bones and old parasites, they can actually forge entire artificial skeletons. This is how every single part of their society is built up. Their vehicles, weapons, and even spaceships are animated skeletons. Their structures are only one of three things: boneyards, slaughtering pits, and graveyards. The boneyards are their primary structure; they are essentially a large stone slab that sometimes have skeletal watch towers around them where they collect and hoard tons of bones. They range from small sized to very large, as in the 'Great Boneyard' that centers every city. Slaughtering pits are where they take captured organisms and lock them up, eventually either killing them and taking their bodies to the graveyards, or until they are fully infected, and become soldiers of the undead. Graveyards are where they bring killed animals/creatures, sentient, or not, and bury them. The purpose of the burial is simply their preferred storage method, rather than leaving them out in the open to attract scavengers which take away their bones. They also take the bodies out of the graves in mass numbers to strip away all the flesh from their bones, and throw what's left (only bones) into boneyards. Boneyards will then, seemingly at random, be visited by the boneyard's owner, where the owner will raise any number and variety of skeletal minions. Flight is the most challenging problem for these skeletal monsters, and it in fact took them several thousands of years in constant feudalism before they learned how to create effective wings. Their only real 'space-ship' is actually an extremely large skeletal monster that bears resemblance to either a vulture, a dragon, and sometimes a gryphon. These 'spaceships' acquire weapons and such by scavenging them from other ships and civilizations. Scavenged weapons are broken up and infused into the 'living' spaceship. Their only for sure method of communication is by a sort of telepathy to other infected organisms; somehow, other empires have entered communications and alliances with the undead. The best theory is that the undead have specialized communicator hosts which are allowed to technically live while their brain is still replaced. A more grim take suggests that these other empires are all already infected, and just waiting for the parasite to officially take control.