The brief third scene of the act serves two purposes: plot and entertainment. In terms of plot, the scene depicts Claudio and Don Pedro fulfilling their promise to Leonato. They pay their penance (an act to show that they are sorry) by mourning Hero for an entire night. Further, because his guilt is so heavy, Claudio promises to mourn Hero every year in the same manner (line 23). For entertainment purposes, the scene also includes a song. But this song is a solemn or serious song: it is a hymn of mourning. The song is positioned in the play as if it is to be sung by Claudio, but actually the song in most instances would be sung by other actors who are also dressed in mourning. The song is addressed to Diana, the Roman goddess of the moon and of virgins. The heaviness of this dark song is, of course, appropriate. But the dark hymn also sets up a nice contrast for the song and dance that conclude the play in the fourth (and final) scene.