U3 The Final Enemy is a rip roaring conclusion to the Sinster Secret of Salt Marsh U series of modules. These modules are dark and pulpy sort of weirdness that you just don't find in today's modules. But there's more to the waters of U3 The Final Enemy.
U3 The Final Enemy is the last in the U series of adventure module for Advanced Dungeons and Dragons; it represents the last of the trinity of adventures in this series. It has a lot to offer in terms of page count and gritty sword and sorcery content. That's one of the secrets of the U series that they have a totally different feel to one another in terms of goals and getting the adventures into the deep end of the events surrounding Saltmarsh.
This module has a totally different tone to many other AD&D first edition modules, not only is there the back drop of the events of the Secrets of Saltmarsh but now with the events of Danger At Dunwater at the PC's backs events are reaching the height of their apex in Saltmarsh's bloody waters. This adventure is for PC's of levels three through five and make no mistake one miss step and your party will be shark bait. Wiki has a pretty damn good break down of the plot from an outside the adventure's perspective; "After identifying the evil creatures, which have massed in force and viciously organized as a threat to the village of Saltmarsh, the player characters have the opportunity to thwart the creatures' plans and ensure the safety of the little town. The Final Enemy is an underwater quest to penetrate the lair of a species of fish-like monstrous humanoids called sahuagin." Right off the bat U3 contains rules for underwater adventuring and combat; and the adventure has the gritty feel of a full blown military operation and investigation with the PC's not only looking into the strengths and weaknesses of the forces that have circled around them since U1 but lizardmen allies of the PC's are there to help move the adventure along. You might think that U3 is only for higher level PC's but that's where you'd be dead wrong there are several potions of water breathing scattered throughout U3 and a wand of polymorph to help the PC's get into their ultimate goal, the underwater lair of the Sahuagin.
There are several key factors with U3 The Final Enemy, this is one of the most Lovecraftian AD&D module of the U series involving a very alien and weird enemy in the form of the sahuagin which over the years I've converted into Deep Ones when running the U series with the Astonishing Swordsmen and Sorcerers of Hyperborea retroclone rpg system. Not only are these creatures incredibly dangerous but the U series had a grim and nasty streak that suited the climate of this underwater adventure location.
Foolish PC's are going to get themselves slaughtered in the underwater environs of the monsters and there are several weird variations of the sahuagin to contend with including the iconic four armed version and several places in the adventure where running can ensure that your PC's are going to live. The adventuring climate of The Final Enemy is one that dovetails very nicely into the back end of L1 The Secret of Bone Hill. The reason I say this is because L1 more of a mini-setting than an adventure, offering several adventure locations, events, and NPC's that can be dropped right into The Final Enemy for even more expansion of the adventure.
The Final Enemy is gritty, action packed, and shows off several features that would come to dominate other modules in different ways. The idea of investigation over combat is one such conceit of U3 and that evil exists right under the PC's very feet in the communities. There are several cross references in U3 that tie the entire plethora of adventures of the U series together into a neat little whole of a mini campaign setting. Some notes about using this adventure series with the Astonishing Swordsmen and Sorcerers of Hyperborea game. U3 presents a very unified and dangerous whole of an adventure. The sahuagin make excellent stand ins for Lovecraft's Deep Ones and they have wide variety of applications outside of this module and can be used to set up a cult within the environs of the Saltmarsh countryside.
After this particular adventure is done with a the PC's are ready to move on, there are several options that can be done to exploit the PC's involvement with the events of the U series and this includes taking the adventure elements and moving them around the countryside of Hyperborea add hock.
There is plenty of room to shift around the locations of the U series of adventures for Hyperborea, this is especially true of the Deep One infested areas of the map of that world. The technology levels, the grittiness, the Weird Tales style of backdrop of AS&SH all fit into the U series of adventure modules.
This very aspect of the Final Enemy is one of the things that allows the U series to blend into the background of the setting. It also makes this one of the must have adventures to convert over to Lamentations of the Flame Princess. The weird pseudo historical setting of the U series enables it to be inserted into everything from LoFP to adding on and building upon several of the Lovecraftian principles of the modules .
U3 The Final Enemy has some incredibly evocative and iconic monsters, encounters, and strange NPC's that can still surprise jaded players of Advanced Dungeons and Dragons. This is one of the other strengths of the U series, that no matter how many times you play it there is more going on under the hood then most folks expect and its different every time. This makes U3 The Final Enemy a very well put together adventure that will have players coming back to the table again and again.