...Hmmmm...
"So - I mentioned the Fall of Xerxes too? So that was hundreds of years ago. Xerxes was the greatest empire in the world, and its king wanted to live forever. And one of his court alchemists had created or summoned a being - the Homunculus Father - that claimed enough alchemical knowledge to grant him that."
"It was a trap. The king and all of Xerxes' million citizens were consumed in an enormous alchemical reaction, which gave the homunculus a physical body, tremendous power, and half a million lives - the other half million went to a slave boy who had befriended the homunculus, the only survivor of Xerxes."
"Both survived to the modern day. The slave went east, to Xing, while the homunculus went west, to a chaotic tangle of warring princedoms. The homunculus allied himself with the princedom that would someday become Amestris."
"The homuculus's involvement in Amestris's rise wasn't widely known, and Amestris developed from a princedom to a military dictatorship. The highest ranked officers were let in on the secret, but no one else."
"The homunculus began to burn through his lives, more quickly than he had hoped, and he wanted even greater power than destroying Xerxes had given him - and Amestris in those hundreds of years developed fifty times the population of Xerxes. So, he promised the highest officers a similar trap as the king of Xerxes: eternal power."
"The Promised Day was a specific date the homunculus needed to make the largest array in history function properly."
"I was recruited to the army about four years before the Promised Day. We found out about it mere months before, but... A lot of what we needed to oppose it was already in place."
"Amestris's government has never been popular, and when I was a child it slaughtered nearly everyone in a rebellious province adjacent to mine. The man who recruited me was a veteran of that war, and it radicalized him - he was a revolutionary, who'd been building his network in secret for years with the primary goal of preventing another massacre. Which, of course, would involve overthrowing the current administration."
"That mostly just set the stage for crowd control and keeping the administration's loyalists and minor alchemical threats - living dolls, chimeras, the like - off the alchemists' backs. The primary physical threat came from the homunculus and the seven lesser homunculi he'd made from the parts of his own mind he wished to cast away - we actually managed to flip two of those, which brings me to your demons' names being very weird because the seven homunculi were Pride, Greed, Envy, Lust, Gluttony, Wrath, and Sloth. First Greed and then Lust both betrayed the homunculus Father in favor of helping us."
"The largest threat came from the nationwide array. We weren't able to stop it from activating - instead, we needed to find a reversal array and set it up to trigger after the nationwide array, changing the array to one that would drain the homunculus of lives rather than feeding him them."
"We succeeded, and we succeeded at killing the enemy homunculi - and at bringing Amestris under somewhat less terrible control without a widespread civil war breaking out."