Using dark magic, dead bodies can be infused with the demonic mockery of a soul, reanimated and forced to do their creator's bidding. Corpses can be reanimated at any stage of degradation, whether newly-killed or only skeletons, but only human and animal flesh can become undead. The animation of inanimate matter can only be achieved through ancient runic magic.
Shambling dead are always slow-moving, and they have no free will of their own, existing only to carry out the bidding of their master. This is because they are powered not by true souls, but by mockeries of souls created by demons. Undead powered by true souls are Revenants, which are faster and stronger. Still, the soulless "shambling dead" are surprisingly strong and difficult to kill, since they must be entirely dismembered and then purged with sunlight or holy power in order to completely cease to exist. In most cases they will also fall if their necromancer master is killed.