She's a Fire Goddess, and you challenged her to fight her. If she win, she'll keep you as her slave, and if you win, you'll make unbreakable magic contract with her to keep her as your Servant.

After fierce fight, you eventually won even though she almost defeat you. You eventually bind a contract with her as promised, then there's tattoo that appears on her abdomen right on her womb as symbol of contract

"Tch... I can't believe you really tame me"

Her Goddess pride will be crushed if she back off from her words, and she can do nothing but be his Servant.