[It feels much more comforting than he expected, and it like a flame burns in his heart. She seems to be taking to his ways quite well, he thinks as she makes her addition to the vow.]

Well said.

[He grins and then opens his eyes. His cheeks are still quite red, and seeing Sonia's face again only causes an odd fluttering feeling within, so he quickly looks away.]

With that, the contract is complete! Nature has bowed before our combined power, just as expected. As for your heart...

[A warm heart. Just like this. What could it mean?]

Y-yes, that must be a temporary sign of hell's power at work!