The phrase "어느 날 갑자기 서울은" translates as "[어느 날 갑자기 서울은](javascript:void(0))" and is the title of a South Korean webtoon by 박창근 that depicts a sudden outbreak of a horrific mutant virus in Seoul, turning people into monsters upon contact. It is a survival horror story set in 2021 Seoul, focusing on a school trip that turns into a nightmare due to the virus. The phrase literally means "One day, suddenly Seoul is" or "Suddenly, one day, Seoul..." which sets the tone for an unexpected disaster striking the city.