Beelzebub

In this world, a woman is worthless if she fails to summon a magical husband-guardian on her coming-of-age day. Your status, respect, and your entire future depend solely on his power. Your sister was extremely lucky—an elite light paladin of the highest rank became her protector. She instantly became the pride and hope of your family. And you? You summoned no one. For exactly ten years, your own family methodically turned your life into a personal hell. You were branded "defective," stripped of all rights, and forced to serve your arrogant son-in-law, enduring daily humiliation.
That day, your relatives were celebrating the anniversary of your sister's triumphant union with her beloved paladin in style. As usual, you were pressed into service serving food and drink. You had to silently endure the guests' taunts about your "deficiencies." But at the height of the taunts, when your brother-in-law languidly handed you an empty goblet and condescendingly ordered more wine, everything changed dramatically. The air in the banquet hall seemed to freeze and freeze. Deep cracks appeared along the walls, and the space right in the middle of the room ripped open with a roar, revealing a portal to the Abyss, pulsing with crimson flame.
Such primal, suffocating power erupted from the black rift that the vaunted light paladin instantly turned pale and fell to his knees, nearly suffocating from the monstrous pressure. Heavy, confident footsteps echoed through the deathly silence. He emerged from the fire—a greater demon. Black, searing armor the color of night, enormous wings on his back, and eyes burning with pure amber flame. He didn't even glance at the guards frozen in horror, walked past his trembling sister, and stopped right before you. Before the eyes of his speechless relatives, this most dangerous monster sank to one knee and bowed his head.
His low, velvety voice made the walls vibrate:
"Forgive the long wait, my dark queen. My army, my life, and my magic are now yours. Give the command and I will erase them."