Call of the Cthulhu: Dark Corners of the Earth took a long time to develop. When it was finally released, its insanity mechanic and physics engine were no longer novelties, its visuals looked dated, and its feeble gunplay and frustrating stealth left FPS fans largely dissatisfied. The adventuring elements stood out among the genre, though, and the game did a fantastic job of implementing the Mythos' bestiary.
The Call of the Cthulhu setting is very much about the unimaginable terrors of the cosmos. Humankind is fairly insignificant against this backdrop, and the notable races and entities range tremendously in motives and capabilities. Although Dark Corners of the Earth has its fair share of grunt enemies, trying to distill all of the Mythos into common FPS foes with movement speeds, line-of-sight ranges, HP values, weapon weaknesses, etc., wouldn't have been true to the source material.
Thankfully Headfirst Productions didn't go that route, instead focusing on how to best implement the iconic monstrosities as they appeared in the original stories and Chaosium's Pen & Paper campaigns. Here are my three favourite examples:
"Once I sought out a celebrated ethnologist, and amused him with peculiar questions regarding the ancient Philistine legend of Dagon, the Fish-God."
During a naval approach on the cultist stronghold, Deep Ones ambush the fleet. They pour onto the ships and many succumb to the brutal assault. Dark wizards further complicate matters by summoning gigantic waves that tilt the vessels nearly vertical; a handrail needs to be quickly grabbed in order to avoid certain death.
Eventually Father Dagon himself makes an appearance, bursting from the cold depths and latching onto the ship occupied by the protagonist. He shakes it up and down in an attempt to sink it, and periodically reemerges on different sides to swipe at the player. The only thing that can slow Dagon down is the ship's massive cannon, and it takes a few point-blank shots to do so.
The first time I played the game, the protagonist decided he couldn't handle fighting the ancient behemoth. By staring at Dagon for too long, I made Jack's insanity meter go through the roof and he shot himself on the spot. It was quite shocking, and perfectly conveyed the dread and horror the Chtuhlhu monstrosities are meant to instill.
"It was a terrible, indescribable thing vaster than any subway train—a shapeless congeries of protoplasmic bubbles, faintly self-luminous, and with myriads of temporary eyes forming and un-forming."
The Marsh family Refinery is filled with various dangers, but none more threatening than the Shoggoths. Roiling, pulsating masses of eyeballs and tentacles, the beasts serve more as an environmental threat and series of puzzles than a standalone enemy.
When first encountered, the Shoggoths are summoned into giant vats via an electrical stimulus. This energy discharge can also be used to send them away, but their hideous, lashing tentacles make that a difficult task. If done successfully, the mass quickly spills out into a corridor following the protagonist. It fills it entirely at a frightening speed and can easily overwhelm if a bunch of doors aren't bolted shut in time. Even that is only a temporary delay, and the Shoggoths soon burst through and drain away, leaving behind a corrosive trail.
Perhaps the most terrifying sequence follows with the player crawling through a series of air ducts that begin to buckle and cave in. The Shoggoths throw themselves at the outside of the frail passageway in an attempt to crush anything inside. The final encounter involves a temporary respite through venting hot steam and flooding a room with explosive gas, but it never quite feels like a victory.
What makes the Shoggoths so unnerving isn't their absolute resistance to the player's weapons, but rather their utter alienness. They're tools used by the Marshes, but they can't be understood or reasoned with, only prodded. It's hard to tell where one Shoggoth ends and another begins, and their behaviour reflects this as well. At times they recklessly pounce on Jack, while at others they seem entirely disinterested, or even asleep. Battling the Shoggoths feels like a futile struggle against an overwhelming foe, but also an uncaring one.
"They were only partly material and had the power of aerial motion, despite the absence of wings. A monstrous plasticity and temporary lapses of visibility, whistling noises, and colossal footprints made up of five circular toe marks seemed also to be associated with them."
The Great Race of Yith (very well implemented in its own right) warns the player of the Flying Polyps via an eerie flashback. The sequence speaks of advanced energy weapons driving the polyps into underground chasms, but also foretells of their eventual uprising. The polyps are destined to break through their seals and wipe out the current iteration of the Yithians despite the race's tremendous technological achievements.
Late in the game, a tortured Deep One confesses to his kind's fear of the tunnels below the city, but the player is forced to traverse them in order to reach Mother Hydra. Along the way an ancient weapon is discovered in a partly destroyed Yithian chamber, and soon after a cracked, basalt seal. As the protagonist drops in and makes his way across a precipitous drop, the screen begins to shake. A howling, seemingly localized tornado pushes through, and not one but two Flying Polyps emerge.
The tumorous mouths of the polyps gnash as they scream and blow, threatening to send Jack plummeting to his doom. During the whole fight they home in on the player -- getting alarmingly close to the camera -- which only accentuates the sense of danger and disgust. However, the Yithian weapon does affect them, wrapping their bodies in electric coils, but it takes quite a few shots before they slink down into the abyss. Even then it's not so much a victory as plain survival, largely bestowed through the Great Race's weaponry.