Alfaðör is an overarching antagonist of Fire Emblem Heroes. He is the superior of Loki and Thórr. He's said to have written the Twilit Runes, a prophecy that encompasses all of time and the destruction of each god of Zenith.
It is mentioned in Xenologue 5 that nothing lies beyond his power.
At the end of Book IV Chapter 12-5, after defeating "Kiran" who supposedly betrayed the Order of Heroes, Alfonse removes the hood revealing another Alfonse. It is this moment that "Alfonse" finally realizes that the reason why he cannot recall any of Alfonse's past life and only has memories of "the World of Steel". The Alfonse present for most of Book IV was actually Kiran who took on his appearance due to the power of the Realm of Dreams. Upon this revelation, Freyja (the main villain of Book IV) appears, revealing that Alfaðör has removed the real Alfonse from existence due to him defeating Hel, making him too dangerous to live.
Alfonse, however, is present upon Kiran and Sharena's awakening, and it's unclear if Freyja was lying or if he'd been revived. Hel's former generals, who were now working for Alfaðör, have arrived to Freyja's realm and recruited the later's generals, who wanted to revive their queen. Meanwhile, Loki has engaged the Heroes in battle once again, possibly following Alfaðör's orders. At the end of the battle, Loki was defeated again and retreated.
At the beginning of Book V, Chapter 10-1, Loki destroys the false Breidablik that Eitri created, and muses to herself that Alfaðör believes that mastering the art of summoning would make Niðavellir too powerful and would disrupt the balance.
Alfaðör may be a reference to the title of Odin in Norse Mythology, All-Father. Odin is the Norse god of healing, death, royalty, the gallows, knowledge, battle, poetry, frenzy, the runic alphabet, and sorcery. He is also known as the father of the thunder god, Thor.