"Notification: Bone Dragon Sario has issued you a challenge. Passing the challenge will allow you to recruit this special individual. Do you accept?"
This Bone Dragon is indeed special.
In such recruitment locations, as long as the City Lord defeats the guarding BOSS, theoretically, they should be able to recruit the high-order troops directly.
Whether or not the troops will actually submit is another issue, but recruitment... high-order troops don't have the right to refuse recruitment.
This seems possible for Bone Dragon Sario.
Or rather, it had hidden its recruitment point. Even if Mu Yuan were to see its information, see the 'awaiting recruitment' text, the recruitment prompt didn't appear until it spoke up.
Next, should he undertake the challenge to gain the Bone Dragon's approval?
No.
It's just a mere Bone Dragon, not worth the effort.
He is the City Lord!