The flavor of this adventure and the lore behind the enemy mobs was fantastic but the actual combat bits were just terrible. This felt like a Tier 2 adventure in disguise. Solo Bosses without Legendary Actions just don't work in 5e (and even those with LAs barely do). The final boss fight is also just way too easy when leveled down, even without the dragon helping out as well. My party of 4 & APL 12 barely took any damage during the entire module. Also way too many fights that aren't even easy.
If I ever happen to run this again, I'm probably gonna rebuild pretty much all of the combat encounters to make them work for 3rd Tier. I'd give this module a higher rating just because the flavor was spot-on, but seeing how this is a dungeon crawl mod I can't in good faith give it anything better than this seeing how core the combat is for these kinda adventures.