BE WARNED - Spoilers for the entire series starting with this post 
Well, turns out I was very, very wrong about this mini-series not being a full on railroad. Still loved the performance and especially loved Brennan's Asmodeus, but wish there had been some actual stakes in the final confrontation instead of a fail forward with complications towards a pre-determined ending. Much prefer meaningful consequences that determine the outcome of the story when players roll dice, especially in a climactic encounter.
EDIT: After some additional time to think about the episode...
Man on man, this was soooo close to being a perfect live-play DnD one-shot. Brennan and all of the players nailed it all series long. And then in the final encounter, maybe because the session had gone on so long, Brennan muddied what the stakes were, what the meaningful result of failure was, and, for me, it really hurt the ending. All props to the players for roleplaying through the emotions of the scene anyhow, but I think this is a great lesson in the critical importance of communicating the consequence of failure to the players.
Brennan's mistake, IMO, was in trying to tie the release of the Primordials to the actions of the PCs in those final 3 rounds and also threatening the total destruction of Exandria, both of which both the audience and the players new was impossible. Because of that, all of the drama around the die rolls fell flat for me (again, if it did for the players, they did an excellent job hiding the fact).
Here is how Brennan could have set it up with the players to really make the end impactful.
"Laerryn, you've done it, in 18 seconds the city will touch down and the Primordials will be banished from Exandria. But there's a problem. Right now, the energy that will be released will send out a wave of destruction hundreds of miles wide, killing all those both in the city and all those who have just recently fled on airships. Enemies have stormed the Abborial Calix to kill you and the party, and you have just 3 rounds to try and minimize the destruction. Here's how it will work. You can make one DC20 Arcana check a round with your action, resulting in the following."
0 Successful Checks - The city and all the airships fleeing will be destroyed.
1 Successful Check - The city and all the people in it will be destroyed, but the airships will survive.
2 Successful Checks - The city will start collapsing, but those in it, including you and your friends, will have a chance to escape with additional checks (this is what actually happened, allowing Cerrit to escape, but not Quay and Laerryn)
3 Successful Checks - The city will be preserved (for now) allowing all those still alive on it to escape.
Most interesting about the above, is that Laerryn would have had an interesting decision to make after making 1 good check, in that killing all of the corrupt officials still in the city and all the devils there may have been a worthy sacrifice.
Please note that I'm really not trying to back-seat DM or throw shade at Brennan. It's amazing what was accomplished in these 4 sessions, and just like in real life, sometimes DMs don't make the best decisions or don't communicate perfectly, especially after a long and draining session. I LOVE this series, but a big part of the reason I watch live-actual play D&D is to learn to be a better DM myself, so the above is really about me processing and learning, not about trying to diminish or crap on anyone involved.
