Unbound (Songs of Chaos, #2) by Michael R. Miller

Sometimes the world needs a little chaos. Holt and Ash saved the kingdom of Feorlen against all odds. Now they are outcasts, alone on an impossible quest to unite the Elder Dragons. But they are children playing in a game of Dragon Lords. Trapped between the riders, servants of Sovereign, and the scourge, even their luck cannot last forever. Their only hope is to advance their bond by any means necessary. In Feorlen, Talia faces a world unaccepting of a rider queen. Her councilors will not heed her warnings of Sovereign. Foreign powers threaten war and bloodshed. Pleas sent to rider headquarters fall on deaf ears. All the while, Sovereign regathers his strength in an ancient fortress. Enthralled cultists swell his ranks. Disillusioned riders flock to his cause. And his unwilling servant Osric Agravain scours the land for dragon eggs. There are new types of magic to be discovered and Sovereign intends to control them all. Only the mysterious half-dragon Rake has a plan to stop Sovereign. To pull it off, he’s going to need a team.