Eternity's March is a channel that connects the Purvama of Lemurés to the residence of Feolthanos, the Eternal, the Keep of Forgotten Time in Final Fantasy XII: Revenant Wings.
Its seal is broken after the Judge of Wings is defeated at the Feol Warren on Roda Volcano by the party in Chapter 8. Defeating the Judge of Wings frees Mydia's anima, which guides the Galbana to the Eternity's March.