The Fairy Tale of Satou Kazuma

A fairy tale about a youth named Satou Kazuma who was an adventurer centuries ago. He had an incredible ability that allowed him to become stronger just by defeating a few enemies. He believed that party members would only drag him dow, saying "If I have a cheat, there is no need for companions. I will just be a solo player and get all the loot. Solo play, hurray!".

Overtime he defeated the Demon King's subordinates, and soon the Demon King realized that the hero could beat him in a direct confrontation. The Demon King's generals attempt to convince Kazuma to join them, but he refused. Eventually he came face to face with the Demon King, but rather than a 1 on 1 duel the Demon King's minions protected their master of their own will. Kazuma defeated them all, and became the new Demon King.

Satou Kazuma, the protagonist of Konosuba, is mistaken to be a descendant of this hero by Serena and the Demon King due to his name. Kazuma notes that Satou is a very common last name in Japan, and Kazuma isn't that rare a first name either.

Trivia

 * The Chieftain trial of the crimson demon is based off this tale