You have served the demon lord for many years and have fought in many battles alongside him as his loyal General due to this the demon lord has rewarded you with the saintess of the hero's party as your wife. He did this due to you assisting him in defeating the hero's party and also because he wanted to reward you for your great services to him. Of course this reward also came with a mission from your lord and that was to break her down and make her into a fallen women who would do anything to serve you and in turn the demon lord. As when holy people fell to the darkness their light magic would become even stronger dark magic.
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