Stories that I found to be suspenseful or to contain other elements of horror and the supernatural:

Stranger is Danger 
Author: AvaCelt
Pairing: Xiumin/Kris, Luhan/Lay
Rating: NC-17
Length: Ongoing
Summary: Wu Fan falls in love and everything falls into place while a serial killer stalks his favorite city in the world.

CHAPTER 1 OF A SERIES. ONLY 4 CHAPTERS CURRENTLY PUBLISHED. I am so desperate for an update for this.

After the Fog Rolls Out
Author: zeerogue
Pairing: DO/Suho, slight!Kai/Luhan, slight!Sehun/Chanyeol
Rating: NC-17
Length: 18 Chapters
Summary: After an incident with his best friend, politician's son, Do Kyungsoo, is sent to live on the coast with his uncle. Heart broken and cast off from society, Kyungsoo feels as if he's lost everything until he meets Lord Suho from the castle up on the cliff. However, no matter how kindhearted and good Lord Suho seems, there's something deep and dark in his eyes and in the sea that surrounds this silent coast. 

DEALS WITH "CERTAIN MAGICAL CREATURES OF THE WATER". I am not one for mermaids, but this fic took a different route. Kyungsoo's character was such a mess! I completely loved how the author wrote him.

Fire Away, Fire Away
Sequel to "A Bad Moon Rising" (a super precious Krisyeol).

Author: jojo_kim
Pairing: Lay/Lu Han, side!Kris/Chanyeol
Rating: M
Length: 21.4k
I couldn't find a summary, but here is the first line