My Girlfriend Is the Strongest Demon DemonGF

Even people who love those novels don’t want to get transported into a world full of danger. But that’s exactly what happened with Shin.
Without warning, his soul was sucked into the world of his own horror short stories. There, monsters roam on every corner, and even main characters rarely have happy endings.
Shin, who was hunted by an evil god of his own creation, has even fewer chances.
Now he, together with other unfortunate transmigrated souls, has to play a terrible game of survival with demons and dark deities. He will have to lie, cheat, murder and bring forth the destruction of his own world in order to stop the ascension of the Fourth God and return home.
Meanwhile, all Shin has to help him is his knowledge and a Contract with one of the (formerly) most powerful demons in existence.

[She] adores you.
[She] will be beautiful for you.
[She] will kill for you.
[She] will be with you forever. No matter what.

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Warnings: gore, suffering, yandere.
No smut & no harem.

The cover art belongs to https://lee-js.artstation.com/projects/xzGnO1
(I'm using it without the author's permission, but if he sees it, I hope he will be forgiving)