Risen Dead

The Risen Dead are the uncountable corpses of the fallen and passed away. As the World Died, the psychic energy of the dying gods swept across the landscape and was absorbed by the empty shells of those that died. This energy reanimated the corpses, which crawled out of cemeteries, battlefields, and other places corpses are commonly found. Now, the risen dead wander the planet, violently striking out at nearby living entities.
The risen dead are the main cause of the destruction of the Old World and civilization with it. Untold millions of undead rose up in every city, town, and village simultaneously and slaughtered the unprepared civilians. Those that survived long enough to mount a resistance retreated to the Citadels and learned to build walls of Radiant Steel, which naturally repel the risen dead.
The majority of risen dead are skeletal constructs animated by psychic energy, though those that had only died recently before the world died still had flesh clinging to them. Whatever animated the corpses also seemed to stop the decomposition of the rest of the flesh, which even a hundred years later still clings to some of the dead. Risen dead are naturally resistant to most weapons, causing radiant steel weaponry to explode in popularity.