The Moon Lord won't spawn with a Celestial Sigil or Celestially Sigil (from fargo souls mod) while using the Builder Essentials mod. Using the celestial sigil caused the messages to appear:
Impending doom approaches...
Moon Lord has awoken!
Impending doom approaches...
Moon Lord has awoken!
Impending doom approaches...
Moon Lord has awoken!
Impending doom approaches...
When the moon lord is awoken, it despawns instantly.
Disabling this mod fixes the issue, but you can't get the build in peace item from the moon lord or use any tools from this mod.