Spookwave

"Spookwave" is the 39th track in the Undertale Soundtrack. It has the same melody as "Spooktune", but with a phaser effect. It is in the second CD found in Napstablook's house. The song will continue playing after exiting their house. This can trigger an event where the protagonist encounters Aaron and Woshua, who will be too scared to fight due to the spooky music.

Trivia

 * "Spookwave" is a reference to the music genre vaporwave.
 * "Spookwave" also sounds very similar to the theme from the Inspector Gadget Animated Series when sped up slightly. This could be a reference to how Vaporwave commonly consists of slowed down samples of other songs.