Spoilers: MAJOR Spoilers for Death Note for everything up to chapter 58 of the manga and episode 25 of the anime. Spoilers for various Cthulhu stories.
Rating: Rated MA or probably NC-17
Warnings: NO UNDERAGE READERS. Extremely violent, including drowning imagery and stings. Extremely gross. Mpreg (not the fluffy kind, but the body horror kind). Sexual activities, including rape, unrealistic depictions of rape's psychological consequences, tentacle rape, violent sex, blood play, and forced impregnation. Scenes of captivity. Includes evil!L and tormented!Light to an extreme degree. Contains cannibalism, necrophilia, vore, guro, diapering and mentions of incest. References to suicide and bestiality. Not my usual fluffy ending.
Summary: Death Note and Cthulhu Mythos crossover. L is not entirely human, and is rapidly becoming less human after a certain event. L sets out to make Light his "bride" by force.
Pairings: LxLight as the main pairing, along with LxWatari and LightxWatari and a minor, one-sided OCxLight and an extremely minor fantasy-only SayuxSachiko.
Additional Notes: All canon Death Note events previous to chapter 58 of the manga or episode 25 of the anime have happened as normal, and then the storyline diverges. Cthulhu Mythos characters are not used at all, and Cthulhu Mythos elements are introduced to the Death Note universe in such a way that these elements are tweaked and changed a bit in order to fit better. Expect to encounter a few completely made-up Cthulhu "facts" in this fanfic. See Wikipedia's Cthulhu Mythos article if you don't know that canon (basically, lots of horror with space/ancient/undersea/religion themes).
Disclaimer: I do not own Death Note or the Cthulhu Mythos, and I do not make any money from these writings. However, the earlier portions of the Cthulhu Mythos canon are in the public domain.
Links to the story: (same content at each link)