I have done a tiny bit of research and thinking for awhile, now here is the story;
Something like a destruction has happened to the world (as you can see things like tombstones and flowers rotting at the start--representing 'death'..etc) and the only way to keep the world safe, is that the Kingdom has to choose someone be the Messiah. A local seamstress at a village, Miku was chosen to be the next Messiah who has to get the nine blessings from the Tower of Ai (as you can see a letter being sent to her), so another nine people (Kaito, Meiko...etc) who were her very best friends since childhood, joined to help her.
They thought that being a messiah was a good thing and if you were chosen it, people would think you're very lucky, btw (which is why Miku looked pretty happy and confident). Being chosen as the messiah is like being chosen to save a princess or something, or do something heroic--well, only to those guys.
After days, they finally managed to get to the Tower of Ai, and Miku, who thought her friends joined her to help, realized that her friends only did it to get the 9 blessings of greed (or so she thinks), and it left her alone and upset, and to go up to the altar alone that was waiting for her, but only when she sees even worse than her best friends betraying;
The truth about being a messiah. The whole thing about the 'good thing about being a messiah' was a total lie.
The nine 'blessings' were just actually curses, and nine people were suppose to sacrifice themselves (I think it's because blood was needed for the chosen next-messiah to become the Messiah. Sorry if it sounds confusing). Kaito and the others have knew about this except Miku, that is why they all sacrificed themselves.
Then, you can see the red candles in the skeletal-dead statues hands, dripping and melting like blood, which shows that Miku's friends are currently going through the 'curse' in pain.
And if you look closely, there is a shadowy ghost there (Maybe it's Neru. Let's just call that ghost Neru. ;) This is also the first thing I noticed when I first watched this music video). This might not be true, but I think 'Neru' was a Messiah, before Miku. And because of her fate that had to be done to become a Messiah, she turned into nothing but a shadowy ghost, so if Miku gets her fate to become the Messiah, she'll become a shadowy ghost too. But like I said, this might not be true.
And Miku? She cries and smiles, as she accepts that she has to have a fate too, to become the Messiah as she was chosen to be it and there was no turning back. The fire ignites, indicating that Miku's friends are finally finished going through the curse and are now dead, and Miku is now pronounced the new Messiah, and 'Neru' disappears, as if she is free now.
