Quake's always a good Lovecraft-Lite game to blast through; classic FPS. You go from military installation into an elder world and face off against a few big and nasties. For the most fun, I recommend playing on Hard Mode. Nothing like being chased by a pair of shamblers down a hall, getting blasted by lightning in the process, and having a fiend you forgot about cornering you in the process, with a chance you also forgot about a bolt-firing trap getting you in the process as well (which can also be used against said pursuers).
However, in one of the expansion packs, having a pet shambler wreak havoc on an elder tower packed with monsters, I loved having that big fuzzy ugly bastard on my side for once, instead of gibbing it. He tore, and blasted, through the ranks like they were made of wet coffee filters.
EDIT:
Always fun to do. In any level where zombies and piranha inhabit the water, cheat yourself a lightning gun and invulnerability, and just fire the gun while you're in the water, and clear out the entire area in a single discharge. Hold tab to watch the kill numbers sky-rocket. Much more entertaining if done during a multiplayer match, and you survive. Even 1 cell of ammo is enough to pull it off, just to take a few down with you (mainly if there's a good number of players online, when it's done).