I only saw this movie once some years ago, that is as much as I can remember:
The protagonist is a teen girl, I think she had a little sister who starts to have nightmares about a creature that she draws in the walls, this creature looks like slenderman in the drawings, the little sister is sent to an asylum because the family thinks that she is getting crazy while in fact there is actually that creature, something happen to the little sister in the asylum, there is a meeting (maybe a party) or something where the teen girl and her friends plus maybe at least one teen boy starts getting haunted by the creature (that has some sort of slenderman skills), there was a scene with a latern acting supernatural, in the end the teen boy (or maybe the protagonist) sacrifice himself trying tu burn the creature in a garage (I remember fire).