Too Late to Retreat
The news about the marching undead quickly spread to all the players. Those who are away from the kingdoms that issued subjugating missions all came flocking to the said kingdoms. The undead were first sighted at a forest at the southeast border of the empire.
The forest that the undead marched on became lifeless as their aura that the undead released absorbed life from the surroundings. The trees became withered husks while the monsters in the forest that the undead came across became part of the legion. Though only a few undead could be said to be top tier, the real danger of the undead is their sheer number.
All the farmlands became barren while the farmers who toiled the soil joined the undead legion. The undead legion did not waver or did not tire so they only marched north. The news of the marching undead even reached the news in the real world as if Atlas Inc. paid all the news outlets around the globe.