Cthulhu is a single-player modification for Half-Life. It contains 65 maps, 21 new monsters and 14 new weapons. Aeons ago, Earth was ruled by The Great Old Ones, beings so alien and terrible that merely seeing them would drive most people mad. Before man's ancestors climbed down from the trees, these being were defeated in a war with The Elder Gods. But their alien nature meant that they could not be slain in any way that we would understand, so they were imprisoned. And so they wait, patient and potent until the stars are right and they shall walk forth once again and wipe mankind from the face of the Earth. You play Ranulf Stafford, an occult investigator in Arkham in the year 1928. Drawn into a nightmare chain of events, you find yourself the only thing standing between mankind and the return of Cthulhu, one of the most dreadful of The Great Old Ones.
To start off, this mod is quite awesome. It has atmosphere, some cool effects, new monsters, mostly well designed levels and a fun, Lovecraft styled story.
Using magic is an interesting experience, since using it will drain sanity and if that hits zero, will kill you. It is absurdly powerful though, so there are times when you have to use it (and times you just want to make cultists explode). The weapons are fun, and look pretty good too.
The monsters are, for the most part, pretty good. Some, however, are merely reskins of Half-Life monsters, and I seem to recall there being some normal Half-Life monsters in there too. Nothing too serious though, just it can take you out of the experience a little sometimes.
The levels are a bit of a mixed bag. For the most part, they're awesome. They're nice to look at, have an interesting design, and are fun. However, a couple levels are rather annoying. I enjoyed Innsmouth, but it was easy to get lost in there. The Ghoul Caves were a cool idea, but it's not too fun to wander around in the dark with the honestly not too good Half-Life torch, and the big puzzle was kind of annoying. Mostly though, the maps are good.
I did notice some graphical glitches though, however, I'm not certain if that was the fault of the mod or my computer. With some weapons (the rifle and the pistol, mainly) sometimes when the cross hairs go over an enemy, the cross hairs are replaced with a black square. A little annoying but nothing too game breaking.
Story-wise, this is nothing too spectacular, definitely feels like a Lovecraft fanfiction, but it's a well done and fun one, so that's no real problem at all, unless you are a fanatical Lovecraft purist.
All in all, it was an excellent trip, that aside from some moments of checking the walk through (helpfully included in the download, props for that), some Half-Life enemies, and annoying maps, is well worth your time and effort.