Seems they're borrowing the term "Mother Crystal" as well as I can't recall its use outside of Eorzea.
I think Joshua was the name of the small blonde boy, and the protagonist is his "shield". I think the boy turned into Phoenix and someone else (possibly the protagonist) turned into Ifrit as the man who is on the ground says "Another fire eikon? Impossible". So I'm guessing there's usually one elemental eikon per crystal (Titan for Earth, Shiva for Ice etc) and then two fire eikons appear which shouldn't be able to happen. The iconography of the title seems to indicate a fight between Ifrit and Phoenix, which makes me think that maybe it wasn't the protagonist that turned into Ifrit. And there are two people shown at the end, the "shield" of Joshua and the man with the marked face. It also looked like two different protagonists fighting in some of the scenes.
I'm wondering if this story is told from two different perspectives.
Much hype for me since this is being produced by Yoshi P and not Nomura. I'm ready for a narrative style change and Yoshi knows his lore and can guide a great story. FF14 for all its game play quirks is one of the best RPG stories out there.