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Nightmare Fuel: Brutal Doom
The Spectres. The partial invisibility is gone. Instead, they are completely invisible except for their glowing red eyes. And these things move fast, and are just as intent on nomming you as their pink cousins.
In addition to their regular fireballs, the Imps can now leap right into your face to claw you to death, just like their counterparts from Doom 3.
The Spider Mastermind and Cyberdemon are even more terrifying. Thought they were intimidating before? How about when every attack they make shakes the earth?
One of the Brutal Doom Mutator side-mods changes the zombified soldiers' grunts and snarls with voice samples, often saying things to antagonise the player. Now imagine hearing things like "You will not escape!" just when you quit the game from main menu.