Canonical Information: Information provided below is entirely canonical.

The Elegy of Emptiness is a song that appears in The Legend of Zelda: Majora's Mask.

Spoiler warning: Plot and/or ending details follow.

It is acquired by completing the Ancient Castle of Ikana and defeating Igos' Servants then Igos Du Ikana. Upon being defeated, Igos Du Ikana's head will teach you the Elegy of Emptiness, which allows Link to form a 1 copy of each of his forms. Each copy if you look closely resembles the body of the soul that gave you the mask: Goron/Darmani, Zora/Maku, Deku/Deku Butler's Son, and Link.

Spoiler warning: Spoilers end up to this point.