"Why would I... why would I agree to that?" The mastermind asked nervously, knowing that she had little chance if she faced off against Eiro directly. But she needed to stall somehow. After all, a royal wouldn't suggest something like that if they didn't know they were going to win.
"Hm, it's simple. If the chimera you create manages to even put a scratch on my son, then I will let you live. Rather, I'm going to help you wipe everyone in that village out." Eiro said bluntly, "But if my son manages to kill your chimera, then you're going to have to cooperate and answer any of my questions. I don't care what you do beyond that, really."
Making use of the mastermind's nervosity, as well as his control over the domain of truth, Eiro made sure that his words seemed as truthful to the monster in front of him as possible. It seemed like her mind was slowly being affected by his words. It didn't take long until she fell for it, even if she did so a bit too easily.