Sam and Dean????!!! Is that Supernatural?!!! haha
http://www.mangago.me/read-manga/magandanai/bt/327206/Ch8/4/
I dont remember much but
-the mc is trapped on a house with another person
-there is a huge field with flowers
-try to leave cause he is bored or something
-find out he was on a coma
-try to return to the house
-the other person turns into a monster/demon.
thank you.
maybe this?
https://www.mangago.me/read-manga/moritat/
This might not be the one you're looking for but it reminds me of "Friends at first".
https://www.mangago.me/read-manga/a_casual_friend/
That was so fucking beautiful! Great story, one of the best dj I've ever read, thank you sensei.