These are some more ideas based off of my previous post about a post evil overlord campaign.
The last free lands the waste lands
During the war against the evil overlord most of the free peoples congregated in the Elven kingdom. This land was protected by the terrible magics of the Elf Queen who kept the evil overlord’s magic at bay. The land was a refuge for the free peoples and the last bastion of hope against the evil overlord. However in the waning days of the war as the free folk were are the cusp of defeating the overloard it released a horrible spell upon these elven kingdoms. This spell caused the land to become barren and the dead to rise to attack the living. The newly slaughtered joined these ranks of the undead. While some fled the overwhelming majority of the inhabitants were killed.
The elf queen worked her magic and sealed the undead to the border of former kingdom saving the rest of the world. The magic has held for a few years, but there are places it fails and the undead spill out into the bordering kingdoms.