Of Last Resort. Princes of the Blood, #1

In a kingdom ravaged by the beasts and terrors of an ancient war, the only hope against them is the army of mixed-blood beings known as the King's Legion: dragoons, shadowmarch, sorcerers, titans... and the Princes of the Blood, nigh-invincible warriors of fearsome magic and strength, their only weakness a need for pure human blood. When his brother runs away, Raffé agrees to undergo the Blooding, but he is a pale imitation of his strong, capable brother.
No one expects him to survive and become a Prince, merely to die to save the rest of his family and preserve honor. But runaways and weaklings are the least of the kingdom's concerns when an ancient evil returns with new tricks, and their only hope lies with knowledge long lost and a fierce bond forged in darkness...