Nightmare is a modpack created by Yoru, a Steam user that gained a lot of attention for this modpack. Why?!? Because it has fundamentally changed the way Left 4 Dead is played. Every golden rule of the game is broken, in exchange for a more terrifying experience. The pack consists of over 180 addons that all work to impede you for the sake of horror.

The collection also contains a series of ambience mods that add soundtracks from Cry Of Fear and other games to enhance the experience. One of these include an addon called "HellChant" that provides hauntingly beautiful orchestra of infected chior singers that can be heard in the distance while playing.

One of the most crucial addons, the Cry Of Fear Color Correction addon, turns the game dark; To the point that your flashlight seems worthless at certain points.

In the midst of this, one of the most impressive mods in the collection, almost appears to be a new special infected of its own. The Howler, taking on the form, (or lack thereof) of a shadow entity that can only be seen in your flashlight. It replaces the Jockey, though its behaviors are completely different.
The creature moves very slowly and cannot, by any means, be killed. Further, it deals a fatal blow, an insant incompacitation (on Easy and Normal) or an instant death (on Advanced and Expert). The best way to avoid this fate, is by simply moving around the entity, (if available to) or in tight corridors, lead the Howler out of your way.

Another important detail, is the sound it makes. It was originally taken from an early build of "SCP: Containment Breach" an indie horror game released in 2012, from SCP-096, upon entering its chamber in the game. That sound always scared me as a kid when I heard it, so hearing it again as an adult, especially in my favorite game, was a real stress-tester.