The Poisoned Crown — Deborah J. Ross

The king is dead, long live the prince, but not for long if his stepmother the Queen Regent has anything to say about it. So he appeals to the one person he can trust, his father's best swordswoman and the king's secret lover. Venise wants nothing more than to bury herself in her grief at the king's death, but her conscience will not allow her to abandon the young man who is so like his father. The only question is whether the two of them can stand against the Queen Regent's black magic.