Adventurers.
Ainz had some fantasies about them.
People who sought out the unknown, venturing around the world. Ainz had certain expectations of this career that embodied the right way of playing the game YGGDRASIL. But after listening to the explanation of the counter lady, he learned that adventuring was a practical and dull career.
In short, adventurers were 'mercenaries who deal with monsters'.
Some parts were similar to Ainz's idealized version of adventurers. They do explore the ruins of countries destroyed by demon gods, and do seek unknown treasures in secret realms; but they are basically monster hunters.