Awesome episode! After 'A Canterlot Wedding' this was probably one of the best episodes so far! Firstly, like every 2-part episode, it is one of the best but I liked the story of this King Sombra and overall the whole idea of the Crystal Empire. It's one probably ther best idea so far with this Empire.
I felt it as if I am watching some good fantasy movie, only in cartoon, it had very nice first half and action. Now, I agree with some posts above that somehow it was rushed - maybe because they started with deep and well thought story and it was great indeedm but then Hasbro realized they won't have time to prolong the defeat over Sombra and made it quicker.
Its quicker end is the only thing that felt not so nice but that's only because after the whole cool part 1, part 2 was expected to be that full and continuous. Still, I loved it, as well as every episode that has Luna <3
The songs were deep, serious and mysterious, I loved them which is why I felt it as if I am watching some good fantasy movie