The Eternal Stairway is a location in Fire Emblem Fates.
The Eternal Stairway is first visited in Chapter 8, where the Corrin and their allies cross though it to get through the Wind Tribe. Corrin complains about how long the staircase is and groans when they are notified that they're only halfway through it. Suddenly, Faceless ambush the Hoshidans. Corrin and Kaze kill the Faceless, only for the magic to drop and reveal them as Wind Tribe members. Iago teleports in, mocks Corrin, knowing he can plot against them with Corrin on the Hoshido side and then teleports out to prevent the Wind Tribe from finding out that it was a ruse.
The Eternal Stairway is visited in Chapter 21, where Corrin and the Nohrian siblings cross though it to get to Hoshido, but unknown to them, Iago summons a large horde of Faceless to ambush them at the Eternal Staircase. Unbeknownst to him, Iago has unintentionally helped Corrin and the Nohrians clear the Eternal Staircase, which was guarded by a group of Hoshidan soldiers to halt the Nohrians' advance. Iago's plan is foiled as the Nohrian army barely manages to break free of the Faceless horde and Lilith sacrifice herself to save Corrin from the Faceless. Although disappointed by the fact that his scheme has failed, Iago expresses content through Lilith's death.
Like in Birthright and Conquest, the Eternal Stairway is visited in Chapter 9, where Corrin and their allies cross though it to get through the Wind Tribe. Suddenly, like in Birthright, Faceless ambush the army. Corrin, Gunter and Kaze kill the Faceless, only for the magic to drop and reveal them as Wind Tribe members. Iago, who was responsible for this, quietly warps out of the scene, pleased with the success of his deceit.