There is a much better thing to do with it

Just like what happened with Rean and Roan, the Universe Foundation Fragment in Fratekla's crystal ball reacted to the pushing force in the old ruins. Soon, a passage without anything blocking their way opened, allowing the demons to pass through. 

Back on the twins' side, Roan continued his talk with the demon soul on the altar. "So, wanna make a deal?"

The demon soul pondered about the situation for a bit. He knew Roan's strength, so he would definitely have the advantage when an Underworld Soul Forcing Link was used between them. However, if Roan knew about the link, he definitely knew about the possibility of taking over the other part's soul with enough time. Not to mention the twins knew about the Foundation Fragments. 

The demon felt like something wasn't right, so he asked, "What kind of deal do you want to do? Is it strength? Submission?"