Three types:
1. One of them is a horror antagonist while the other is one of the protagonists
2. Both are trying to survive a horror scenario
3. Both are the horror  antagonists of the scenario

Inspired by @Kohai-Karma 's oneshots Experiment

A.    A laboratory in the out-curst of a secluded town had made horrifying experiments under the cover of being a hospital for the citizens. They ended up creating an intelligent creature that was able to adopt and learn quickly. 

1. The scientist take on 'interns' to help around the hospital but ended up finding out about the creature when working the night shift...

2. The scientist had made different versions of the creatures and one came out wrong/weak. Another one of the experiments got protective/possessive and would attack those who hurt the being would end up nearly or dead.

3. Those who come into the hospital in critical condition would be given to the creature as food and said that they couldn't save them nor give back the bodies. No one questioned it much though. One day, a patient had enough will to beg for mercy and the creature decided to use  them as a plaything and window to the outside world instead of meal. 

4. A scientist/doctor that had been moved to the hospital to help with the creature. It got very attached to them and tried to play with them or keep them from leaving. To the point that they used their abilities to immobilize the doctor.

5. After the place was closed down and abandoned, the creature would kill those stupid enough to go inside to steal items (drugs, blankets, equipment, etc.). A group of teens entered the hospital planning to do just that. The creature found them and decided to play with them. Or a teen that had left home for some reason (your choice) and went into the hospital for cover from the weather. They heard a noise but thought it was an animal so stayed in the front. The creature watched them angrily wanting to play without leaving the hospital -the person could easily run out- and observed them, questioning why they looked so thin... 

B.    A family of cannibals lived just outside of the village. The villagers were terrified of them since they were rich and could easily buy the ground they walk on so they respect and fear them. Servants would disappear and they kept hiring desperate people but they all ended up in a meal after a while. 

1. Someone new came into town looking for work and saw the add for the manor around. They got the job however the heir seem to act very strange: getting close to their neck, kissing their hand, and stealing some of their items (they live in the manor).

C.    A family had been near perfection to all those who knew of them. However they would practically brainwash them into perfection and those outside that became tortured or cursed into dolls that obeyed the person that loved them, only freed when the person allowed them to be.

1. A dysfunctional family came to visit them. One of the child didn't want to come at all and the heir(s) of the family got attached and tried to woo them even as children. The parents (normal) thought this was cute and asked if the child could stay with them. The perfects agreed however the child continued to try and hide from everyone in the manor but when caught the family would give punishments for misbehaving

This is a threesome ship but I'm using killermare as base

After buying a new house, the couple had noticed strange things happening. At first, Killer believed it was ghost while Cross was skeptical. The longer they spent the more it looked like this third entity seemed attached Cross and Killer, though leaning to Cross. Killer then convinced him to do some weird ritual which resulted in Nightmare being summon and starting to claim them.