I actually saw a video which, most likely, became the inspiration for this. Still think Applebloom or Scootaloo would've made a better Frodo
Getting to the matter at hand - the concept is quite striking; while dark isn't exactly MLP style, it fits the premise nicely, allowing the background to gently play its part in the whole, giving the sense of dread to the final moments of Sweetie Belle and Applebloom's journey. A mature and more realistic vision seems befitting to such work.
Of course originality is always put under debate; as I said - this piece is practically taken out of the video I mentioned above. Thus that make it unoriginal? Thus using a scene from a movie/book means one can't count this as "something new"? The image is unique, in its own way, but that still doesn't help shaking off the feeling "I already saw that".
The style you used fares much better; the dark and gloomy background perfectly mirrors the shadows that cover Mordor, while raining balls of fire present an excellent guide to the entrance of Mount Doom. Both Applebloom and Sweetie Belle are done in a more mature technique then one sees in source material; undoubtfully a fitting choice, playing homeage to the more striking theme the piece represents. However, their faces seem like those of hobbits more so then ponies, being mostly flat. A minor thing, that could be labelled as "personal preference", of course and yet... this is MLP after all.
The impact itself is overwhelming, in more ways then one - the burning sky lights up the darkness of Sauron's lands, giving to view a picture of two, brave fillies that are determined to complete their mission, with the fate of Equastria/Middle-Earth resting in their hooves. That determination practically beams from Applebloom, who obviously has her eyes set on the goal out front.
The rather empty background perfectly indicates the ash-filled waste that is Mordor with a single, black structure towering over it, housing the fiery, all-seeing eye. All this gives an extra boost to the desperate feeling that slaps you in the face the instant you lock your eyes onto this piece.
Sweetie Belle and Applebloom are not passive to all this, with the former showing the fatigue of the burden that was placed on her neck. Similary, the yellow filly is not about to leave her friend and being the ray of hope in this forsaken place, she's ready to see this through to teh bitter end.
Without doubt, while there is some room for improvement, this piece does its job splendidly, bringing forth the experience the viewer will not soon forget. And even if this was "done before", the impact remains the same, which is admirable.