Jiang Lan's father had already sent word, giving him free rein. In other words... the Prime Minister's Mansion planned to use this opportunity to make a move against the Dao Cang Sword Sect.
Su Qinghan's master, the Sword Immortal, was the closed-door disciple of the Sect Leader from two generations prior at the Dao Cang Sword Sect. Although she only held a nominal position there now, she would absolutely not stand by and do nothing about this.
Thus, there was another layer of meaning to the message from Jiang Lan's father: he wanted to use Su Qinghan to test the Sword Immortal.
'Which means, if I don't touch Su Qinghan now, she'll actually be in danger.'
Why?
'The original Jiang Lan had no cultivation to speak of. And aside from a bit of a fixation on cleanliness, he was known for never turning away a beautiful woman.'
'By some "twist of fate," he was about to take the Sword Immortal's only disciple, Su Qinghan, as a concubine. How could he possibly resist touching her?'
