Casper and Nova is a song that plays in the spinoff series Adventure Time: Fionna & Cake in the episode of the same name. The first minute plays when Fionna and Cake wake up in what they think is a newly magic Fionna-World.
The next section, starting at 1:00, plays later in the episode when a Simon-possessed Shermy and Beth explore the library in future Ooo before being attacked by monsters.
The last section, starting at 1:56, plays when a Simon-possessed Shermy begs Beth to continue the story they are reading in order to learn the whereabouts of The Crown in what Simon thinks is an alternate version of Ooo.