The Infectious Groove is a magic song/spell used by Haruo Weyland, a devil summoning rock star who started to use it when he ran out of inspiration for his music. He stole Personas in order to make a song everyone would love, because they would all hear what they want.
Persona 2: Innocent SinEdit
Tatsuya Suou and the rest of his group get called in to investigate a mysterious rise in violence at Karukozaka High School. There, they find a demon attempting to steal a boy's Persona. They defeat it, but later find out he was just one of many demons summoned by Weyland to steal Personas so he could analyze what they like and steal the spark for his Infectious Groove. However, since the Persona is a manifestation of a person's inner self, when one is stolen the person becomes a feral animal, explaining the increase in violence. He explains to them that he could no longer find inspiration for his songs, so he used the stolen Personas to help figure out what people liked, and used what he took to make the Infectious Groove. When all his demons are defeated, he surrenders the stolen Personas to Tatsuya's party. It's later revealed that he got his COMP from Tamaizu Nirasawa, who was manipulating him for his own ends.