The Bastard Princess and the Doomsday Dragon

Lyra Vesland is the king's bastard daughter, with royal and dragon blood, and she spent years shut in an old tower. She is sent to the frontier as a peace bride. The wedding is a ritual meant to use her blood to wake Morgath, the last black dragon. Morgath wakes and kneels to Lyra instead of to the crown, and the king and the nobles who wanted the dragon for themselves turn on her. She refuses to be anyone's weapon and allies with the northern commander Kael North, gathering soldiers, exiles and freed slaves into an army against the old bloodline order. Her legitimate sister Seraphine wakes a white dragon and moves for the throne, which puts the two of them on opposite sides. Every time Lyra uses dragonfire it takes a memory from her, including what she remembers of Kael. She gives up the path of conquest and joins with Seraphine against the king and the plot behind him, and they set about abolishing slavery and burning the dragon-bone throne.