"Into the Unknown" is built around a duet between Idina Menzel's Queen Elsa and a siren call voiced by the Norwegian singer-songwriter Aurora.
The big ballad number is set three years after Elsa's coronation, when she hears a strange sound coming from the north. At first, Elsa tries to ignore the voice, but once she recognizes it is calling her, she feels compelled to follow the sound. The rest of the movie tells the story of Elsa's journey "into the unknown."
Aurora's siren's call, a distinctive sequence of four notes, is woven by Frozen 2 composer Christophe Beck throughout the film's score. The call is derived from the Dies Irae, a Latin hymn best known for being used by Roman Catholics Mass for the dead. Dies Irae (meaning "Day of Anger" or "Day of Wrath") speaks of the Day of Judgment in fearsome terms; it then appeals to Jesus for mercy.
Aurora delivers her haunting vocal in a manner inspired by Scandinavian kulning, which is a shepherdess's call from Scandinavia for their livestock to come home.
A pop-rock cover was recorded by Panic! at the Disco. Their version was released as the soundtrack's lead single on November 4, 2019.
A video was released for Panic! at the Disco's version. The clip cuts between shots from Frozen 2 and footage of frontman Brendon Urie crooning the tune in an empty theater.
Aurora recorded her vocals in an old English church in London. "The room was beautiful, round, and it carried my voice to infinity and beyond without any extra reverb," she recalled to Billboard.