City of Dawn (1)

Iris said nothing, so the demon lord continued. "I won't take you to bed against your will nor do I plan to take lovers behind your back. This, I promise you."

The silence that filled between them grew heavy like a fog. It wasn't like Iris was oblivious to what Lord Kieran was saying. She and Chrysanthe had made sure to learn more about the mating rune that was put on her back.

Aside from the fact that it was one of the spells created by the Black Mage himself, it was said that the mating rune would force those who were bound by it to be together whether they liked it or not. 

Neither she nor Lord Kieran could take on another lover because the moment they were coupled with another being that wasn't their mate, their lover would experience immense pain before dying.