This was an animated children’s movie that I saw when I was younger. It was probably 2010-2012 when I saw it; my guess is that it was made in the early 2000s.
In this movie a girl (she might be a princess, I think I vaguely remember her living in a castle) runs into the forest. Many monsters come out of the woods. These monsters come to attack the girl. They walk on two legs. I think their skin is green, but other than that I can’t remember any distinguishing features that would classify them as monsters.
Then a boy comes and starts singing. He sings something about the night and the dark and not being scared. The monsters cover their ears and run away because they hate singing. They go down to some sort of underground fortress. The boy says something to the girl about how he does this a lot.
Later on the boy and the girl sneak down to the fortress for some reason. I also remember that this movie was not animated by Disney and it had a strange and somewhat long title. Also I remember this movie scaring me but I was little and I don’t think that has any presidence.
The Princess and the Goblin
The secret of kells?
https://www.imdb.com/title/tt0485601/?ref_=tt_rec_tt
Even though the title is very short, and the years don’t match, I’m pretty sure this is Trolls. https://www.imdb.com/title/tt1679335/
I think the main character Poppy is a princess. Branch is the singing boy character. There is also a scene reminiscent of a Holocaust/concentration in it that still makes me sad/ill when I think about it, and some sad backstory with Branch’s parents, so I can definitely see it scaring a kid.
If it’s not the same movie, they have very similar plots!