This opening sequence is a tracking shot of a cgi snake from close as it leaves the skull to wide as it slithers through the grave yard before changing the audiences focus to the statue. the camera then glides up towards the clouds as the title appears, slow music is played creating a dark feel to the film showing that something bad is happening or is about to happen. this title sequence sets the colour scheme for the rest of the film as dark greens and blues these colours signify the dark themes in the film and the bad moods of the characters who are in the moody teen phase of there life, The use of these colours also suggest coldness and night time establishing that the film is starting in an exterior place at a late time suggesting bad things will happen as this is the sort of style adopted by many horror films.
this video shows the opening again with the vfx removed showing how a studio was used to obtain dark lighting and how images were layered to create a realistic environment it seems like this has been shot on a stedicam or crane as there is a smooth floating feel to it. this makes it feel un-natural and dream like which as you find out later is exactly what it is. this establishes that the film will have something to do with a dream.
this opening scene shows the period in which the film is set by having world war two planes bombing Britan with a family seeking shelter. once the children are on the train the music turned more mystical hinting towards the later events.