Overlord: Vampire Princess of the Lost Country.

Overlord: Vampire Princess of the Lost Country - A Strange Tale of the Absolute Ruler

Author: Kugane Maruyama

Illustrator: so-bin

EDIT: The moonrunes pointing to Ainz read "Formerly Unlucky Salaryman, but one who goes on a love-love journey for two with a cute blonde girl." So I think it's pretty much confirmed that it's an expansion of the side story.