The Queen’s Dog by N.L. Holmes

In the Bronze Age Syrian kingdom of Ugarit, a eunuch slave mistakenly believes his mistress the queen loves him, so when asked to spy on her under threat of punishment, he finds a way to avoid it. When she shows she cares nothing for him, he justifies reporting her disloyal activities but can’t escape agonies of conscience. As her sentence of divorce turns into execution, he decides he must avenge her and expurgate his own guilt.