It is really rare to see a 2nd season that is as good as the 1st. I don't really think this is a sequel. It is just the 2nd part of the same drama.
It was a great experience. I really love fantasy and historical dramas. But it had two problems I think. First, the overall really complecated story of Naksu/Bu Yeon. I couldn't understand it and I don't believe it helped the plot. Second, this season was really fast paced. Especially the end was super hurried. As slow was the 1st, so fast was the 2nd. I believe a lot of things needed more time. It was rushed in the end. If both seasons had 15 episodes, it would be better. The 1st needed less episodes and the 2nd more.
All in all, it is a great drama and we rarely see this type of k-dramas. In the bonus, amazing special efects. Also, Lee Jae Wook was great and had great chemistry with everyone.