A friend and I were having a discussion about weird/crazy things we've seen in media, and I can't remember the title of (what I think is) a manga. In it, this girl is an Eldritch abomination, I think she's an alien that has come to earth, but she might be from another plain of existence, and she meets this boy who has a mental problem that makes him see monsters all around him. She's the first person he hasn't seen this way, and they fall in love, or he falls in love with her. By the end, she or her babies end up taking over the world.

An eroge called Saya no Uta

Thanks a lot. I was having a hard time figuring this out!