I think Zemniaz fell before Aeor. I can't prove it, but I can throw massive doubt on the information we've been relying on to conclude that Aeor fell first.
Brennan's said twice now that Aeor was the last flying city to fall. At first I thought maybe Brennan was leaning on qualifiers like "last great city." I wanted to sort out whether Brennan was retconning this, but considering that he's worked SO closely with Matt to prepare Downfall, I couldn't help but feel like we must have missed something.
Zemniaz has been discussed very few times in the stream. In C2E110, the Nein discussed the city after the dinner with Trent Ikithon (Beau's conspiracy board scene):
Other than that it fell sometime during the Calamity, Matt didn't specify when Zemniaz crashed into the Zemni Fields.
Then In C2E125, the Nein interrogated Halas in an effort to learn more about Cognouza and Aeor. He's not very cooperative given that the Nein didn't help him out of the gem when they first found him.
At the beginning of the conversation, Halas claims that Aeor fell "before my time." Straightforward information that no one did an insight check on. After that, Caleb suggests that if Halas answers three questions, he'll see about answering more questions in "different circumstances," implying that he'll help Halas out of the soul gem he's trapped in.
Halas said that he's from Zemniaz, he grew into the belief that he could only rely on himself, and then he abandoned Exandria -- presumably to live in his Heirloom Sphere. He also makes no mention of the city's destruction, but that makes sense given that the Nein were asking about Aeor.
Now, if we stopped there, then we could conclude that Zemniaz fell during the calamity after Aeor. But you know what?
Halas is a fucking liar.
In Episode 125, Matt responded to Caleb's 19 insight check:
Explorer's Guide to Wildemount, Chapter 1: Story of Wildemount, Section 1: History of Wildemount, Subsection: Age of Arcanum, Page 12:
Powerful Archmages such as Vecna the Whispered One and Halas Lutagran began to carve their paths into history during this era of unbridled magical experimentation.
"Before my time" my ass. He was already a prominent mage in the Age of Arcanum, before the Calamity even started, and certainly before Aeor fell. This fucking fucker lied to us and we all took him at his word.
I don't have an answer whether Matt intended for that to be misinformation or if it was an unintentional contradiction. The Explorer's Guide to Wildemount came out on March 17, 2020 and Episode 125 aired on February 11, 2021 -- nearly a year later. Episode 83 (when the Nein met Halas) aired on October 31, 2019. I don't think that Matt would make that mistake given that he had been done or nearly done writing EGtW when Halas first showed up. I'm leaning on the side that it was probably intentional misinformation. After all, Halas was exceptionally distrusting, and he couldn't know why anyone would ask him about Aeor. He knew Aeor had been destroyed by the gods; do you think a hermit archmage is going to trust random bozos who found his gem and started pestering him about it?
Information from Halas can't be trusted, so it doesn't inform us one way or the other. Thus, I wouldn't say that Brennan is retconning so much as clarifying an unknown. And giving us more reason to realize Halas is a fucking liar.
In conclusion, if any of the lore you're relying on came from Halas, maybe add an asterisk to it.
