Wikipedia
"Rattleballs" is the forty-sixth episode of the fifth season of the American animated television series Adventure Time. The episode was written and storyboarded by Andy Ristaino and Cole Sanchez, from a story by Kent Osborne, Pendleton Ward, Jack Pendarvis, Adam Muto, and Moynihan. It originally aired on Cartoon Network on January 27, 2014. The episode guest stars Rainn Wilson as the eponymous character.
The series follows the adventures of Finn (voiced by Jeremy Shada), a human boy, and his best friend and adoptive brother Jake (voiced by John DiMaggio), a dog with magical powers to change shape and grow and shrink at will. In this episode, Finn meets an old sword-fighting robot named Rattleballs (voiced by Rainn Wilson), and learns that Rattleballs was part of an elite force made by Bubblegum long ago to protect the Candy Kingdom; however, the group became too destructive and so they were all destroyed, save for Rattleballs. After a short skirmish, Rattleballs proves that he is not a threat, so Bubblegum relents and does not destroy him.
During the recording of "Rattleballs", Wilson brought his son to the session; DiMaggio noted it was odd because both he and Shada were excited to meet the actor due to his role in the American sitcom The Office, whereas Wilson and his son were in turn excited to meet them, because they were both fans of the show. The episode was viewed by 2.213 million viewers, and Oliver Sava of The A.V. Club applauded the episode for providing Rattleballs with a solid debut.