The Imperial Colosseum Incident was a duel between Sorcerer King Ainz Ooal Gown and Martial Lord Go Gin at the Grand Arena. While only meant to be a public spectacle, it became an incident with major political ramifications.
After finally securing E-Rantel from the Re-Estize Kingdom, Ainz Ooal Gown and the Sorcerer Kingdom moved into the city to establish a strong foothold in the New World. Ainz used his supernatural abilities to benefit the citizens of E-Rantel, intending to change their perceptions about him. However, despite ruling the city with benevolence, Ainz felt something was amiss. During a stroll across the city, he came upon the Adventurer's Guild. Once inside, he saw much of the building has become empty. It was there he learned that his good intentions has left many people deprived of jobs, particularly the Adventurers.
This did not sit well for Ainz who had great respect towards the Adventurers who reminded him of the treasured times he had playing YGGDRASIL with his guild members. After a discussion with Moknach, the Sorcerer King decided to rectify the matter by changing the systems of the Adventurers, from hired mercenaries that kill monsters to explorers of the unknown.