Can anybody tell me the name or author of this fantasy book/series?

[arthuriurilli]

I think it's a trilogy, but there may be more books in the series, though I have only read the first two. I read them in 2003-2004, and I don't remember the titles or author and would like to find them to read again. The main character in the first one is a prince, who is away from his castle when it is attacked and his family killed. He is an expert with throwing knives. The attackers are winged creatures that have been bred by three witches across a vast ocean, where they were exiled 300(?) years earlier. Some of the magic is called 'the vagaries' but I don't remember if that is the good magic or bad. The prince eventually kills the leader of the winged creatures and becomes their leader, and is forced to procreate with one of the witches who bears a son. The plot in the second book is about towers the witches are building out of a special stone that they are mixing with blood to try to control the magic.

If anybody can figure it out from this, please post an answer!!