To be honest, the real issue I had with this book was that the flow in the series was interrupted by MacRieve. Oh, I did catch the bit with Nix about the h in MacRieve. It's just that one mention of a name does not a story make.
Series complaint over.
The tale of Lanthe and Thronos was a long time coming - we'd gotten hints of it in sister Sabine's book, brief hints in Carrow's book, etc. Since the other books involving characters on the island took place in pretty short order, the wait for this one was a bit...ridiculous. And, if you're not possessing of a good memory, you'd feel lost, unless you went back and read those others. Am tempted to do that myself, and I generally remember details enough that I wasn't lost, just irritated that the flow was disrupted.
This couple...I felt for. Due to his family, they were essentially kept apart for centuries. Now, Nix has intervened. Funny really, that she used The Order to play matchmaker. In any case, he grabs her and essentially they're thrown together on some grand adventure whereupon they get to learn hard truths without familial interruption. It was never stated, but I got the feeling his father, and later his brother, had targeted her specifically, likely because she WAS his mate.
One thing that does disturb me - and this is the second book where it happens - is the concept of the surviving mate killing themselves after their spouse dies, even when children are involved. I didn't like it in Revenge of the Sith (refer it to pulling a Padme), and don't like it in my books. I get the sense of loss, etc. I do. I just can't quite understand why a parent would leave their kid(s) orphaned like that.