The following information is from the Adventure Timecomic book series. It may not be part of the same canon or continuity as the animated series.
Adventure Time Vol. 6: Masked Mayhem is the sixth graphic novel in the Adventure Time series.
In this sixth Adventure Time original graphic novel, BMO and Jake are getting ready for the biggest costume party ever—they’ve received invites from everyone! As they party-hop, they realize that each party has one thing in common: a prankster. Detective BMO is on the case with the help of Jake, his sidekick, to figure out what’s going on. Join them as they travel all across Ooo to find who the culprit is, while still enjoying some fun times with friends.