Criteria: Horror Related Content and Level of Darkness.

Scary Skyrim Retexture by chocolambot

No esp needed. Just textures. This makes the ghouls of the night look
like they have come straight out of hell--'tis a thing of beauty.

Sweet Mother - The Night Mother Improvement by 7inus

The Night Mother looks creepy(ish), and she is suppose to. However, with this mod, she has achieved her maximum beauty and will send any adventurer screaming in terror. Download today!

More Bony Skeletons by jerminator1977

Those toy little skeletons laying all around needed a grungy make over.
With this texture replacer (no esp!) those things look creepy--as they should.

Heart of the Beast by NsJones

This texture overhaul also includes sounds--which are great! Installing this mod will make werewolf encounters quite horrific. The beasts of legend look monstrous and don't sound like little puppies anymore (and makes you look and sound more ferocious as a werewolf too) .

NOTE: There are some customization options, but we recommend the "Alpha Werewolf" options.

Ethereal Cosmos by Gamwich

This mod doesn't make anything darker really, but it does beautifully enhance the night-sky. (That, and it is GhostAgent's favorite sky texture mod--which is the real reason why it is on our list.) The only way this mod may add difficulty is that you will most likely be surprise-attacked more at night since you will be constantly mesmerized by the awe inspiring view up above.

NOTE: There are multiple file versions available and the one we recommend using is: "Ethereal Cosmos Twinkle Subdued". Because it rocks.

Revamped Ghosts by mindboggles

It is mind boggling (see author's name above) that this most wondrous of ghost textures wasn't already included in game. It is absolutely fantastic to hear the ghost and barely be able to see him. You will frantically look about for eyes piercing through the darkness trying to pin point his location. This mod is a must for increased paranoia. We highly recommend it.

Dark Fantasy Overhaul by AceeQ

This will add some nice change to the scenery if you have gotten bored with vanilla. Bethesda did an absolutely amazing job in creating their world. There are times it is simply breathtaking. That said, it feels (and looks) like Skyrim is stuck in a perpetual spring. This mod sets a very different mood--a very dreary one. We would say that it takes the jump right out of Skyrim's spring and places it in a fall setting. It doesn't over do anything like so many others out there, but simply turns happiness into sorrow--love it. Our hats off to AceeQ for a fantastic job.

Unique Dragon Priest Masks by skyrimaguas

The original Dragon Priests masks look "alright", but all look the same except for some changes in their colors. This delicious nugget is a complete make over (the ladies should appreciate that term) of all the masks giving them a more horror themed look--which we like very much. The best part is that it is modular so you can include only the ones you like.

OR

Apophysis Dragon Priest Masks by pondcypress

More than likely, the Unique Dragon Priest Masks mod won't be everyone's cup of tea. A great alternative is this mod. It changes the looks of each mask, but in a more fantasy style. However, like the above mod it is modular so you can pick and choose. Combine the two together to achieve the collection you want!