Saishou Hosa to Kurokishi no Keiyaku Kekkon to Rikon to Sonogo: Henkyou no Chi de Futari wa Fuufu wo Yarinaosu

Heroic black knight Lina can fell monsters and slay dragons with ease, but when it comes to romance, she's still a total rookie! After dozens of dates ending in failure, she's ready to give up on her quest of finding a husband, even if the idea of a family is an alluring one. The hitch? Marriage is a requirement for all black knights! You see, the ability to use old magic—essential for safely slaying dragons—is a rare gift, so there's staggering pressure for black knights to pass their power on to the next generation. Worse yet, Lina's in immediate danger without a partner to bless her armor. But it's going to take the right person... Enter the prime minister's aide, Joshua Granwell, a handsome and talented white mage who's a perfect match for her mana. He'd make the ideal spouse for Lina—except she does not want to marry a noble. A contractual marriage might save Lina's life, but surviving the marriage hereafter will be a challenge all its own!

(Sourec: J-Novel Club)