Half-Life: Cthulhu - An Unspeakable Mod
An older and apparantly notorious mod I missed, with the "Unspeakable Remod" overhaul. Just the usual Lovecraftian trope of "P.I. pursues a cult and gets drawn into a conspiracy to release ancient baddies", albeit solved almost exclusively by shooting things rather than investigation. A pretty hefty amount of content for a fanmade mod - much longer than the better-looking but extremely short Source mod - and a lot of custom assets, though there was still a little too much recycling of vanilla HL models, and frankly some of the models just looked comical. Whilst the environments are reasonably atmospheric and spooky, some were downright irritating to navigate due the rather useless vanilla flashlight. Perhaps a more helpful but finite light source would've been a better choice. There were a fair few complaints about the difficulty of the mod, and whilst at medium difficulty I thought it was reasonably easy early on, the ever-increasing number of hitscan enemies can get to be a bit much.
There are a few conventional weapons - melee, revolver, shotgun, rifle and tommy gun - supplemented by molotovs, dynamite, and a handful of spells. The balance between saving ammo and choosing the correct weapon for the job was quite tightly regulated in the initial sections of the game, which are populated by plodding zombies, knife-wielding cultists plus the odd firearm-toting fanatic, and lightning-fast ghouls which can tear your head off in a couple of strikes. Very soon this was thrown out the window, and I almost exclusively used the shotgun's alt fire, dumping a few of my hundreds of surplus revolver rounds into environmental hazards. Meanwhile, the team seem to have attempted to make the "sanity meter" a critical resource all on its own, refilled only via completing objectives or defeating bosses, but over my entire playthrough I only used it once, to refill my health for the final boss. At the start of the game you're told the meter can be damaged by looking into things you shouldn't, but I never noticed anything aside from spells having any effect on it.
Levels are a mix of the solidly linear with maze-like structures sprawled across multiple maps. There can be a fair amount of backtracking in one or two of these, and I can imagine some players missing something critical from one end of the area and spending an hour running around fruitlessly hitting the use button on strange walls. Still, I can think of far worse examples in DN3D mods. There were also a few frustrating moments where the game would detect that you'd picked up an item or performed an action when you'd done no such thing, blocking progress and forcing a reload from much earlier, or causing you to miss out on a new spell.
Overall, not sure I can recommend the mod even for Lovecraft fans, as it's more of an interesting coat of paint for an FPS joyride than an attempt an recreating the terror of the classic stories. Perhaps Half-Life isn't really the best base for this sort of experience, though I have to say They Hunger - and yes, even Afraid of Monsters - do a better job of using the aging GoldSrc engine to evoke tension. TH also does a better job of segueing from slow-paced horror to all-out action. If you have HL installed already, it might be worth a whirl though.