If Astarion's Book of Hungers is the player-facing add-on for Heroes of Faerun, than Netheril's Fall is the slightly more beefy add-on for Adventures in Faerun. The new digital "DLC" for the Forgotten Realms book is a gazetteer for the lost kingdom of Netheril, with an overview of the fallen kingdom, a look at two of its cities (one flying city, Eileanar, and the landbound city Conch), some magically-themed environmental hazards, and a short collection of mini-adventures in the style of the Dungeon Master's Guide. Netheril's Fall is functionally a mini-gazetteer, albeit not quite as beefy as the five gazetteers found in Adventures in Faerun that detail various regions.
At first glance, it's easy to see why Netheril's Fall wasn't included in Adventures in Faerun. Netheril is a fallen empire and having an adventure in Netheril will likely require the use of time gates (which are conveniently explained in the supplement, with two adventures dedicated to traveling and crossing through a Time Gate. I'll note that the adventures in this book feel much more specific than what we saw in Adventures in Faerun or the 2024 Dungeon Master's Guide. While the adventures in other books could be "plugged" into any level-appropriate campaign, these adventures feel much more narrow in scope and cover specific locations or events found in the DLC. There's a few adventures in Adventures in Faerun that cover locales seen in the gazetteer, but there aren't very many that cover a specific event. I don't mind the "one page adventure" format, but I'm not sure it works with every adventure type. If an adventure sends players into a mad wizard's attempted ascent into godhood, I want a little more meat than a few encounters and a resolution of "well, the players will have time to explore the city before the ritual fails."
Another criticism I have of Netheril's Fall is that Netheril doesn't feel nearly as fantastical as an ancient high-magic society should. Given that Adventures in Faerun did a very find job of elevating the Forgotten Realms beyond its generic high fantasy trappings, I feel like a bit more care could have really sold Netheril as more fantastical in nature than what we got. If we could have gotten a few more pages about Netheril's culture, I feel like Netheril would have been more appealing as a place that DMs would want to incorporate into their campaigns. As presented, there are a few cool ideas, but even the phaerimm (one of my favorite D&D monsters) feel a bit flat compared to what we would have gotten.
I was critical of Astarion's Book of Hunger's price point, and while I feel that $15 is still too high for Netheril's Fall, there's a lot more "content" compared to that book. Generally speaking, some adventure content, a gazetteer, and some monster statblocks is probably a bit more valuable than one new species and a collection of feats, although I'm probably a bit biased towards DM content since I'm usually the one behind the DM screen. I personally would had preferred a $10 price point for this DLC, but others may feel that the price point of these are more fair.
All in all, I feel a bit underwhelmed by Netheril's Fall. Netheril seems like a very cool part of Forgotten Realms lore, but it's not given full justice here. I know there's some other Netheril content out there from past editions, and I encourage people to check those out on DMs Guild if they want a real deep dive into this lost empire.
If you're going to call it Netheril Rising, you need one of those guys to take a ground city and be in preparation to make it fly 
