The ability to transform other beings into undead

What exactly happened to the ancient undead Empire? Alex suspected that there were hidden secrets unknown to most. 

The previous undead lord was a native of the Eldoria continent and had many friends, yet he still disappeared in the end. His talents were undoubtedly not inferior to Alex's. 

At his level, Alex couldn't even begin to imagine how powerful the undead Empire must have been—probably filled with Emperor level beings as common as dogs and Sovereign levels everywhere.

Influenced by her past life's perspectives, "disappearance" often implied death. Otherwise, the vast undead Empire wouldn't have crumbled. 

For instance, when Alex was trapped in the Nytheria continent, the Undead City continued to operate normally because all undead knew their queen was still alive. 

When Eldrin mentioned that the previous undead lord had disappeared, Alex was more inclined to believe he had died.