It's basically a story of good versus evil, with foreign invaders coming to land to kill and enslave and bring physical and moral corruption with them, but I think that's a little too simplistic. It's written like an older, mythological tale of changes brought to a land by ancient evil. Some of the parallels will be obvious as it's loosely based on New World cultures and history, sometimes a little too closely, but for the most part it works. Combined with the writing style, the book actually reminds me a lot of The Silmarillion. I enjoyed it quite a bit.