Deep Horizon is an adventure for 13th-level characters from Wizards of the Coast.

The Bottom Line
Deep Horizon is a perfectly OK set of encounters for a 13th-level group, with the potential to be much more if the GM is willing to put in the work. I acquired my copy second hand at a discount and consider it barely worth the money.

There are challenging combats and worthy foes, as well as some potentially interesting roleplaying opportunities involving both a new race and a rival band of surface adventurers.

There is no story as such, just a situation into which the characters are placed (note that some might consider this a "pro"). Plot hooks and NPC motivations are a bit bland. The adventure blithely introduces a race whose weakest member would be considered a mighty warrior in the armies of most game worlds. It assumes the existence of an Underdark-type world beneath the world, as well as the Drow (though as no Drow actually appear in the adventure you can substitute an appropriate race from your own campaign).

The details (spoilers abound)
Deep Horizon starts off with the PCs learning of earthquakes and sightings of strange creatures by night. Investigation reveals a fissure in the earth where none previously was, and further exploration leads them to encounters with the Desmodu.

The Desmodu are a new race of Large bat-like monstrous humanoids. This adventure was published, I believe, during 3.0, and seems to lack the idea of Level Adjustment. However, the Desmodu have 12 racial hit dice (!) and the adventure suggests that if they are used as a PC race, they be introduced when 14th-level characters are appropriate, so that's basically a +2 LA. They deserve it with several very handy special abilities. As with any high-HD race, you immediately wonder why they don't rule the world. But this is more a problem with D&D in general than with this adventure.

The Desmodu have been isolated from the rest of the world for centuries, since they diverted a lava flow to destroy a Drow city against whom they were fighting a losing war. The lava flows also cut the Desmodu off from outside contact, but the recent upheavals have ended that isolation. However, they are now beset by enemies (including former allies) who seek to destroy them.

The backstory and current political situation among the Desmodu is probably the most interesting thing in the entire adventure, but it is given short shrift. This adventure could have been much improved by spending a page or two on important Desmodu leaders of the various factions and their interactions. Instead, three factions are described (one of which provides many of the opponents in the adventure) but not really detailed.

The bulk of the adventure is made up of encounter descriptions in two areas outside the Desmodu enclave. One is the destroyed Drow city, which is being systematically looted by several Beholders and their slaves. The other is the home of the Desmodu's former allies and trading partners (a group of Noble Salamanders) who are now conspiring with the Beholders to kill and enslave the Desmodu.

And here is my real logical problem with this adventure. The basic Desmodu is a 12 Hit Die creature. 12 Hit Dice. And many of the Desmodu NPCs have class levels on top of that. And there are hundreds of them in the enclave. It is expected that the PCs (4-6 characters of 13th level) will defeat all the beholders and salamanders. How can those same villains hope to have a prayer of defeating hundreds of Desmodu with flying mounts? I just don't see the threat, which makes the whole thing kind of fall down.

There is also a possible murder mystery, but the actual culprits (the other party of surface dwellers) will be obvious to the PCs. Convincing the Desmodu of the truth is the hard part.

There are several interesting combat encounters in the adventure. The most interesting are against Desmodu, actually, as they are portrayed as smart, practical combatants with sound tactics. They make good use of their special abilities, including a long range blindsight that could really vex dwarf PCs used to having the advantage in the dark.

I initially planned to give this adventure 2 stars rather than 3, but in fact there is a lot of potential in this adventure, and the race it introduces. But it will take plenty of work from the DM to extract that potential.