To unapologetically steal from Mike Rugnetta: Here's an idea — No matter if it's divided into three chunks, something that only comprises one story cannot be considered a trilogy, and it will always be weaker for having been unceremoniously hacked to pieces. Usually in the middle. But how do we know if something is a set of stories or just a single one? Well, the answer here is to ask, "What's the tension? Where does it resolve?" A story always needs a tension; this is what gives it us sense of beginning, middle and end: A tension is introduced, played out, and resolved.
By this logic, The Umbral Aristocracy Trilogy isn't a trilogy at all. At best, it's a duology, but even that's stretching it. From start to finish, the tension comes in the form of the question, "Hey, what's up with this map?" and that tension is only resolved at the end of the third module. The 'trilogy' only has one tension and true to form, it's the middle chunk that suffers.
When you think of it in these terms, it's clear to see why Beneath the City of the Dead feels lacklustre in spite of some pretty great individual moments, particularly in its bonus objectives. Helping out a dwarven gravedigger with what seems like a scavenging dog ripping up corpses, but turns out to secretly be a pack of ghouls? Nice twist! Helping a wererat's ghostly sister find closure? Cool scenario! But neither feel connected in any real way to the main adventure. They feel tacked on.
And really, could they be any other way? Beneath the City of the Dead has a lot going for it; the combat encounters are actually really neat in the way they make use of tight spaces and numbers to feel claustrophobic and powerful. (Only one fight involving swords, armour and portraiture misses its mark.) But it can't tell any other story beyond: You come into this crypt. Oh hey, you made it through the crypt.
Beneath the City of the Dead is a flavourful but otherwise by the numbers dungeon crawl. It lacks compelling story, has a hum-drum quality to the entire proceeding and cannot help but feel like it's padding time within a trilogy that is only one story rather than a trio of stories that tie together. While it is undoubtedly a victim of the structure of the entire trilogy it also cannot escape the sense that it could have been much more.
[2 of 5 Stars!]