Today the, official Fire Emblem Twitter shared some info about the mysterious 6th country and two members of the Four Hounds.
However, for convenience’s sake, we’ll go through it anyway.
First, a description of Gradlon:
Gradlon is currently a land in ruins. It’s the home of the Fell Dragon who rose from the depths of the earth and tried to invade the continent of Elyos 1000 years ago. Through the power of the Divine Dragons and the Emblem Rings, it now lies sealed beneath the seabed of Lythos.
You may recognise the depiction of Gradlon as the dark landmass surrounding Lythos towards the centre of the mural. However, like the description states: Gradlon now lies submerged and doesn’t actually look like the mural (could be an artist’s interpretation, for instance).
Next, a profile for Zephia:
Zephia (VA: Rika Fukami) is the leader of the Four Hounds, who plot to resurrect the Fell Dragon. She’s a descendant of the Demon Dragon Clan who live for millenia and has been working in the shadows to collect the Emblem Rings.
Zephia appeared along with the other Four Hounds in the recent story trailer. Her map sprite shows her riding a wyvern, wielding a sword (I think). She also appears in the map where everything’s on fire.