Elise was staggered by the sudden offer that Satan request in exchange of helping them. Satan wanted her to become the Queen of Hell, his heir!
"No." Ian was the one to answer. "You want her to be the Queen of Hell when she has a blood of an angel? Your people won't accept her."
One should be happy to be appointed as a Queen but the job of a Queen of Hell is not something anyone could handle. Ian trust that Elise had the dignity and intellect to become a Queen but not a Queen of Hell. She had a heart of an angel, even purer than most angels not to mention the danger of becoming the Queen where she had to subdue the entire Hell. He knew not anyone would agree in her being the Queen even if she had the blood for it. The danger might be something he could handle but Ian didn't want to risk it for Elise to stay near danger.
He only wanted a peaceful life with Elise, away from all the damned danger. Once he had lost his mother and he won't lose Elise.