The Diabolical Dive is an adventure for level 5 to 10 characters set in Avernus. It is the third and final part of the Behind Infernal Lines trilogy. The characters must breach the infernal fortress of Plagueshield Point, which is at the bottom of the River Styx, and retrieve the plans that the Hellriders used when they attacked Avernus.

I’m not sure why the characters want to retrieve the plans of a failed attack, but the young cleric Dara wants them. And, so the characters will do what she wants.

The plot first requires the characters to consult the drow wizard Respen for a way into the fortress; Respen was once its commander but is now trapped in hell along with Plagueshield Point due to a bungled ritual. This section of the adventure typically requires the players to engage in some role-playing with the wizard’s imp familiar before gaining the wizard’s aid. Alternatively, the characters could try to infiltrate his tower to steal a method of getting in, but – as they don’t know what it is – I can’t see why anyone would try this. What is most peculiar about this section is that it has an estimated duration of 90 minutes. If you stretched out the role-playing, I could see it possibly reaching 30 minutes, but that requires a lot of improvising from the DM. There are three pages of content that most groups won’t encounter.

The expedition to Plagueshield Point is simpler. You’ve got a dungeon plan, details of the creatures and their locations, and tactical notes. There are three features I thought were excellent features: A Perception check while entering the fortress can reveal additional ways of entering which affect play, the diabolical fate of the drow wizard’s apprentice, and finally, the actions of the base’s commander when attacked.

Apart from that, it’s all quite straightforward, perhaps too much so. I felt that there was a lack of exciting features to explore in the fortress, which is not helped by the mundane attacks of most of the devils. These lower-tier devils are very simple: a melee attack, perhaps a ranged attack, and a few resistances, and that’s it. The corrupted drow apprentice either provides better opposition or an attractive role-playing opportunity. He’s the highlight of this area.

One of the bonus objectives presents the players with a puzzle. I gave this to my players to solve, and after three attempts and about fifteen minutes of play, they gave up. It relies on you arranging pictures to show the history of Plagueshield Point. However, there’s no help anywhere else in the adventure to relate this history, nor is it clear from the pictures what the order is.

As with all the adventures in this trilogy, it does a poor job of explaining what the goal is. At least there’s a sidebar that explains the points that should have been covered in previous adventures but weren’t. Unfortunately, though you know the characters are after the plans of the Hellrider attacks, their location in Plagueshield Point are not explained to the players, nor the form they take. Was it intended that Respen tell you? I can’t see any reason he’d know. And, if you ask the devils of the fortress, not one of them knows for what you’re looking. If the players should be surprised by their location, better groundwork needs to lead up to the revelation.

The Diabolical Dive works as an adventure. You have to add some details, so the players are more informed, but the adventure flow works, and it has interesting challenges. However, it doesn’t quite intrigue me as Infernal Insurgency does, despite being much better constructed. Your players are likely to enjoy it.