Brian McKenzie’s The Horror Under the Mountain is an ambitious product. Presented as a 42-page pdf, it presents an adventure for characters of levels 14-16 that takes them from the remote human settlement of Ice Reach to the depths of the mind-flayer city of Bal Crek in an attempt to stop the mind flayers from unleashing the tarrasque on the surface world. It’s a great idea for an adventure.

A large part of the adventure is devoted to describing Ice Reach. It’s a strange place, where the plentiful supply of granite from the nearby quarry and the lack of wood means the buildings are mostly made of granite. The buildings feel entirely too large, and the compound itself is huge. The inhabitants have grey skin and eyes, due to the effects of a dangerous fungus. As a player, it’s the sort of place that would immediately make me suspicious and, yes, Everything Is Not As It Seems.

There are a couple of transitional encounter locations (a goliath embassy, the lair of a silver dragon) that the party visit before the great city of Bak’Crek is described. I don’t find the city of mind flayers as exciting and interesting as I felt I should have. It’s good that it includes a number of embassies from other Underdark powers, but for the most part the adventure just describes monsters without really providing interesting features to really play up its alien nature. My favourite encounter involves a pair of devils who offer to play a game of chance with the characters. The gaming implement? A Deck of Many Things! That’s a fantastic encounter, right there! I wish there were more of them.

One of the bigger problems with the adventure is that a lot of the interesting details are in the background and may not be discovered by the players. In particular, there’s a complete lack of information on the main threat: the summoning of the tarrasque. In fact, it’s hardly mentioned. (Using the tarrasque as a McGuffin? That doesn’t happen often!) The best parts of the adventure involve what the mind flayers have done to dominate Ice Reach and its surroundings, which is properly unsettling and horrifying.

The adventure has a lot of text, and I find the font to be too small for me to read it easily. The formatting of the adventure is mostly good, but there are times where it looks just a little wrong. One area that doesn’t work so well is numbering both the buildings and their individual rooms in Ice Reach; switching between numbers and letters would have worked better and caused less confusion when looking up descriptions.

I think The Horror Under the Mountain shows a lot of promise and some very good ideas, but the construction feels a little clunky and unpolished. There’s no doubt that a DM could do some interesting things with this adventure, but it will need some attention to fully bring it to life.