Sarah had defeated a beast and eaten a spirit which made her strong. She continued her travel and ran into a monster town where different monsters live in peace as one, she decided to stay in the village for some time. During her stay in the town, she learnt that the town was made by an adventurer who later became a Demonlord.
(Sarah's Mind: Wait, what is a Demonlord?)
[Google]: A Demonlord is an individual who is seen as a Vessel of Destruction. To become a Demonlord, one has to kill up to 250,000 people while A Celestial Sage is seen as a Vessel of Creation. To become a Celestial Sage, one has to be as strong as two demonlords and should have defeated two demonlords. The Demon King is the strongest of the Demonlords while the Supreme Deity is the strongest of the Celestial Sages. Then there are Magical Beasts, these are beasts with power close to that of Demonlords and Celestial Sages. The spirit you devoured is part of the Magical Beasts. Finally, there is the Administrative which heads the System. The System was created by a Dragon but the System killed him because he possessed more power than the System.