The demon that makes people sing and dance is named Sweet.
Sweet appears in “Once More, with Feeling,” the musical episode of Buffy the Vampire Slayer. Dawn Summers accidentally summons him using a mystical talisman she believes she has obtained legitimately. Once he arrives in Sunnydale, people begin expressing their private thoughts through songs and compulsive dancing.
The effect is more dangerous than a normal musical performance. Characters reveal secrets they would usually hide, while the supernatural singing exposes romantic feelings, fears, and guilt. The spell also causes some people to dance with increasing intensity, and Xander and Anya’s duet becomes one of the episode’s most important emotional moments.
Sweet is often confused with other Buffyverse demons because many recurring demons have distinctive names and comic personalities. Clem is a gentle demon with floppy ears, Halfrek is a vengeance demon, and D’Hoffryn is the powerful leader who recruits vengeance demons. None of those characters causes Sunnydale’s musical outbreak; that supernatural disturbance belongs specifically to Sweet.