Oath of fealty - Elizabeth Moon

Paladin's Legacy #1

Thanks to Paks’s courage and sacrifice, the long-vanished heir to the half-elven kingdom of Lyonya has been revealed as Kieri Phelan, a formidable mercenary captain who earned a title and enemies in the neighboring kingdom of Tsaia. Now, as Kieri ascends a throne he never sought, he must come to terms with his own half-elven heritage while protecting his new kingdom from his old enemies and those he has not yet discovered.

Meanwhile, in Tsaia, Prince Mikeli prepares for his own coronation. But when an assassination attempt nearly succeeds, Mikeli suddenly faces the threat of a coup. Acting swiftly, Mikeli strikes at the powerful family behind the attack: the Verrakaien, magelords possessing ancient sorcery, steeped in death and evil. Mikeli’s survival and that of Tsaia depend on the only Verrakai whose magery is not tainted with innocent blood.
 
  Two kings stand at a pivotal point in the history of their worlds. For dark forces are gathering against them, knit in a secret conspiracy more sinister and far more ancient than they can imagine. And even Paks may find her gods-given magic and peerless fighting skills stretched to the limit and beyond.

Epic / High Fantasy