Last October 10, 2017, Warpportal has announced an upcoming game event titled "A Nightmare on Prontera Street" in Ragnarok Online 2: Advent of Valkyrie. The said game event will begin on October 18, 2017. The announcement did not include much details about the upcoming event. However, the announcement hinted that it will be a quest driven event where players are expected to follow a questline, complete quests and earn rewards. As for the rewards, the announcement did not include specifics - aside from the words "fantastic rewards".
Read Warpportal's announcemnt here: A Nightmare on Prontera Street.